Why does Walmart (SYMBOL:WMT) have its Q1 2015 end April 30 instead of March 30 The Walmart 10-Q form covers the period for the three months ending April 30th while other companies post their first quarter reports for the three months ending March 30th. Why does Walmart end one month earlier than other companies? How are companies regulated in this regard?
A December fiscal year end, which gives a first quarter of three months ending March 30, aligns the fiscal and the tax year. This can be very convenient and in the United States is sometimes required. In addition, some regulated firms like banks are required to prepare documents on calendar quarters regardless of the month of their fiscal year end, and it is often cheaper and easier to have them be synchronized rather than close the books additionally for regulators. 

Firms may be motivated to alter their fiscal years so that they can better serve their customers or investors. For example, some banks have a fiscal year end not in December so that customers needing to do financial transactions at the end of their year could do so without the bank simultaneously having to close their own books. For many years Morgan Stanley did this. Walmart may be doing this because December is the Christmas shopping season in the United States and management wants to focus on that critical shopping period without the distraction of accounting issues. Additionally, if some shopping comes in after the holiday (say if Hanukkah comes late or because of a rash of belated Festivus shopping), it might split a single shopping season into different fiscal years which could be undesirable for investors trying to compare retailer performance. 

I don't want to lean too hard on any of these explanations for Walmart's timing. If I recall correctly from looking in Compustat a few years ago more than 80 percent of US listed firms have a December FYE and this didn't vary a huge amount across industries.

UPDATE:
I went into Compustat and I redid the table of retail, banking, and other firms showing the months of fiscal year end the last 12 months (8/2014-7/2015). 
I was surprised to see that retail was indeed vastly (>20 times) more likely to have a January FYE than a typical publicly listed firm in 2015.  The results appear in the figure below. 

Source: Compustat

This gives credence to the hypothesis that as a retailer with significant holiday sales, Walmart may have a January FYE to highlight those results.