29 February

29 February is the 60th day of a leap year. There are 306 days remaining until the end of the year. This day only occurs every four years, in years that are divisible by four (i.e. 1992, 1996, 2000, 2004, or 2008.) The only exception to this rule is centuries that are not divisible by 400, such as 1900.

This date would have occurred twice throughout the Harry Potter series:
 * In 1992, during Harry Potter's first year at Hogwarts School of Witchcraft and Wizardry.
 * In 1996, during Harry Potter's fifth year at Hogwarts School of Witchcraft and Wizardry.