When did "resilvering" large telescope mirrors actually refer to aluminization, and why was it necessary? This answer nicely summarizes the general picture of historical use of silver for mid to large size telescope mirrors, and the shift to aluminum once good quality vacuum pumps and chambers suitable for this process were available. A freshly evaporated, atomically clean aluminum surface will can sometimes violently react with oxygen, so this has to be done under high vacuum, and vented carefully with inert gas first.

When did the term "resilvering" generally start to actually refer to aluminum coating, and not silver? Was that actually the case by the 1960's or 1970's? 

From what I remember, evaporated aluminum will form a protective oxide layer ("native oxide") of the order of 10A thick within seconds or minutes that is self limiting - no further oxygen can penetrate this layer. 

If this is so, why was re-silvering actually necessary? The cost, risk, and downtime associated with removing the primary and other mirrors every few years sounds like something you wouldn't do unless absolutely necessary.
Amateur telescope and mirror maker here. Not sure if I qualify as a "citable source" but anyway, here it is:

All metals will eventually tarnish. It may take a long time, but it will happen. The process is not entirely chemical always. Sometimes it's purely mechanical (abrasion). Other times it's in between. Surface phenomena are complex.

Even gold-coated mirrors will tarnish. You could use gold for infrared observations. But no matter how much care you take of it, eventually you will have to re-coat it. Sure, gold lasts a pretty long time, but not even gold is forever.

Aluminum is covered with a natural layer of colorless sapphire (aluminum oxide), which protects the metal from further destruction - sapphire is very hard and very inert chemically. But oxygen still diffuses through sapphire (albeit extremely slowly), and so do other corrosion agents like sulphur oxides, marine salt, etc.

So, yes, replacing silver with aluminum was an improvement. But not even aluminum is forever. In practice, you will maybe double the lifetime of the reflective layer when you migrate from silver to aluminum.

It should be noted that silver is still used. Aluminum is most popular nowadays, but you'll still find silver-coated mirrors (its reflectivity is higher than aluminum), gold-coated (for infrared), etc.

There are further coating techniques, like layering silicon oxide on top of metal, which further improve the lifetime of any kind of metal.