Usually it is April 15th. The exception is when April 15th falls on a weekend (as it did this year in 2011), in which case the deadline to file taxes is extended to the following Monday.
April 23, 1564. The date is speculative based on his baptism on April 26 of that year. And also of course it was the day he died and St. George's Day as well.
September 16 (1810). During the early hours of that day, father Miguel Hidalgo started the revolt that eventually became the War of Independence against Spanish colonial forces.