Why do stock exchanges close at night?
Here are some plausible reasons why markets might continue to close: I believe there are electronic exchanges that run continuously,  but the older ones don't want to change their practices since some people may have strategies which  (claim they) are based on this behavior so there would be a lot of unhappy people if it was altered. The pause doesn't seem to do any harm. There are alternatives if you dislike it. Don't try to fix what isn't broken. Most stocks are not actively trades by lots of people.   When you buy or sell a stock the price is set by the “order book” – that is the other people looking to trade in the given stock at the same time. Without a large number of active traders, it is very likely the pricing system will break down and result in widely changing prices second by second.   Therefore for the market to work well, it need most people to be trading at the same time.
Typically the settlement price for a financial instrument (such as AAPL stock) underlying a derivative contract is determined from the average price of trading in that instrument during some short time window specified by the exchange offering the derivative.  (Read the fine print on your contract to learn the exact date and time of that settlement period.) Because it's in an exchange's best interest to appear as fair as possible, the exchange will in general pick a high-volume period of time -- such as the close of trading on the expiry date -- in which to determine the settlement price. Now, the expiry date/time may be different from the last time at which the option can be traded, which may be different from the underlying settlement time.  For example, most US equity options currently expire on the Saturday following the third Friday of the month, whereas they can last be traded at end-of-day on the third Friday of the month, and the settlement period may be at a slightly different time on the third Friday of the month.  (Again, read the contract to know for sure.)  Moreover, your broker may demand to know whether you plan to exercise the option at an even earlier date/time. So, to answer your question: After-hours trading can only affect the settlement price of an underlying instrument if the exchange in question decides that the settlement period should happen during after-hours trading.  But since no exchange that wants to stay in business would possibly do that, the answer is no. Contract expiry time, contract exercise time, final contract trading time, and underlying settlement time may all fall at different dates/times.  The important one for your question is settlement time. Depending on the size of the donation, you may be able to reduce taxes further by donating appreciated assets, such as stock or fund shares that have gone up a lot. That lets you dodge the capital gains tax on redeeming the shares, and if you're donating to a tax-exempt organization they don't have to pay that tax either.  And as @JoeTaxpayer has confirmed, you still get to deduct the current value of the donation, not just the basis value of those shares. So if you're donating anyway, this comes close to being Free Money in exchange for some slightly annoying paperwork. (Yet another benefit of long-term investing!) Of course folks in the top brackets sometimes set up their own tax-exempt foundations so they can decouple taking the tax break from deciding what to do with the donation. There are several reasons why this may happen and I will update as I get more information from you. Volumes on that stock look low (supposing that they are either in a factor between 1s and 1000s) so it could well be that there was no volume on that day. If no trades occur then open, high and low are meaningless as they are statistics based on trades that occur that day and no trades occur. Remember that there has to be volume to get a price. The stock may have been frozen by either the exchange or the company for the day. This could be for various reasons including to prevent some illegal activity. In that case no trades were made because the market for that stock was closed. Another possibility is that all trades that day were cancelled by the exchange. The exchange may cancel all trades if there is unusual, potentially fraudulent or other illegal activity on the stock. In this case the last price for that day existed but was rolled back by the exchange and never occurred. This is a rare situation. Although I can't find any holidays on that date it is possible that this is how your data provider marks market holidays. It would be valid to ignore the data in that case as being from a non-market day. I cannot tell if this is possible without knowing exchange information. There is a possibility that some data providers don't receive data for a day or that it gets corrupted. It may be worth checking another source to ensure the integrity of the data that you are receiving. Whichever reason is true, the data provider has made the close equal to the previous day's close as no price movements occurred. Strictly the closing price is the price of the last trade made for that day and so should be null (and open, high and low should be null too and not 0 otherwise the price change on day is very large!). Therefore, to keep integrity, you have a few choices: You can find the NYSE holiday dates listed on the exchange's own site (already linked in answer above), which should obviously be consulted as the most reliable source; they are also published in an article that I have written here: NYSE Holidays 2016, which provides additional information about traditions and events that can be expected to lead to unscheduled closures, and closed dates for holidays that are day-of-month rather than date specific (e.g. President's Day and Memorial Day). NYSE Holidays are not quite identical to those for the Chicago Mercantile Exchange, though most US stock exchange dates are the same. Also, note that both the Merc (via the Globex platform) and NYSE Arca have different normal cash sessions and trading hours to the New York Stock Exchange. "You can infer some of the answers to your questions from the BATS exchange's market data page and its associated help page.  (I'm pretty sure a page like this exists on each stock exchange's website; BATS just happens to be the one I'm used to looking at.) The Matched Volume section refers to all trades on a given date that took place on ""lit"" exchanges; that is, where a public protected US stock exchange's matching engine helped a buyer and a seller find each other.  Because there are exactly 11 such exchanges in existence, it's easy to show 100% of the matched volume broken down into 11 rows. The FINRA & TRF Volume section refers to all trades on a given date that took place on ""non-lit"" exchanges.  These types of trades include dark pool volume and any other trade that is not required to take place in public but is required to be reported (the R in TRF) to FINRA.  There are three venues via which these trades may be reported to FINRA -- NASDAQ's, NYSE's, and FINRA's own ADF.  They're all operated under the purview of FINRA, so the fact that they're ""located at"" NASDAQ or NYSE is a red herring.  (For example, from the volume data it's clear that the NASDAQ facility does not only handle NASDAQ-listed (Tape C) securities, nor does the NYSE facility only handle NYSE-listed (Tape A) securities or anything like that.) The number of institutions reporting to each of the TRFs is large -- many more than the 11 public exchanges -- so the TRF data is not broken down further.  (Also I think the whole point of the TRFs is to report in secret.)  I don't know enough details to say why the NASDTRF has always handled more reporting volume than the other two facilities.  Of course, since we can't see inside the TRF reporting anyway, it's sort of a moot point." When I place an order with Scottrade I also have to specify if I am wanting to trade outside of normal hours. I was recently at the National Physical Laboratory in the UK and discussing exactly this.  By January 2018 financial institutes will be legally required to time stamp all transactions (including high frequency trades) with a UTC time code.  Today this is almost exclusively done using GPS satellite time and as the OP states - these can be spoofed but also are vulnerable to certain weather conditions.  One of the many innovations of NPL in their recent diversification includes sending ‘time’ into the city by  optical fibre which is ‘gold standard and cannot be tampered with’.  Very interesting topic ( I thought )! from what i understand, which is not much, some companies use some of their own company shares as securitisation for loans. If the share price decreases, the security in the loan decreases, which means the company would need to find new capital. It can create a vicious cycle if the fall in share price is the result of operational concerns.