In the internal chronology Tolkien meticulously constructed for his legendarium, Frodo Baggins and Gollum reach the Cracks of Doom on March 25, 3019 of the Third Age, ending Sauron's reign. The date's proximity to the medieval European new year and the Feast of the Annunciation was almost certainly deliberate, layering Tolkien's Catholic sensibility of redemption and rebirth into his fictional calendar.
Tolkien was a philologist obsessed with internal consistency, and he built elaborate appendices of dates, moon phases, and travel times to make Middle-earth's chronology withstand scrutiny. That he chose March 25, the very day medieval Christendom marked as the beginning of salvation history through the Annunciation, for the destruction of the Ring and the fall of the Dark Lord is almost certainly no coincidence.
Today, March 25 is informally celebrated by fans worldwide as 'Tolkien Reading Day,' an occasion promoted since 2003 by the Tolkien Society to encourage public readings of his work. It is a rare case of a fictional date achieving quasi-holiday status, discussed earnestly alongside real historical anniversaries.