|MadSci Network: Other|
There is never a February 30th. There will be a February 29th in the year 2000, though. The question probably arises because the year 2000 exercises all of the rules for determining leap years, even the most rarely needed rule. The complete set of rules is as follows:
Since 2000 is a multiple of 4, of 100, and of 400, all three rules come into play. Their effect is to make 2000 a leap year, but just a regular 29-day leap year, not some sort of supercharged leap year with 30 days.
For more information, see the web pages at Amherst or CSIRO. The adoption of the modern ("Gregorian") calendar includes some interesting anecdotes, such as a one-time month with only 21 days, and a conversion delayed by religious differences, so that for centuries different countries used different calendars.
Try the links in the MadSci Library for more information on Other.