Did the Mayans observe leap years?
The modern sidereal year is something like 365.242198 days long (that’s what it was when i was in high school, it may have been refined a bit more since).
The Mayan calendar was a bunch of calendars all running concurrently (they kept track of the lunar cycles, the Venus cycle, etc), but their solar calendar was 365 days long, no account being made for the fractional day. The five days added to the 360 day calendar weren’t assigned to a specific month; analogously, the Egyptians later added five days to their 360-day solar calendar, which had exactly 12 months of 30 days each.
http://en.wikipedia.org/wiki/Maya_calendar#Long_Count