Hearth's Warming is an annual holiday in Equestria that celebrates its founding. It is similar to the real-world and also in-universe holiday of Christmas in that it is celebrated during winter and has similar traditions including decorating trees and exchanging gifts.
Origin
Spike narrates through pageant retelling that long ago, before the rule of Princess Celestia, Earth Ponies, Unicorns, and Pegasi were divided by hatred and cared only for the welfare of their own kind. In exchange for demanding food grown by the Earth ponies, Pegasi managed the weather and unicorns raised and lowered the sun and moon. However, the tense peace between the three tribes is broken by a mysterious blizzard.
With the Earth ponies unable to tend the land and the Pegasi and unicorns unable to stop the blizzard, the three tribes fight and argue even more, and the blizzard grows stronger. A summit between the three tribes' leaders is eventually held, but they are unable to reach an agreement. Chancellor Puddinghead and Smart Cookie of the Earth ponies, Commander Hurricane and Private Pansy of the Pegasi, and Princess Platinum and Clover the Clever of the unicorns set out to find a new land.
When the three pairs all meet in the same place, they argue over whom had first stake over the new land, and another blizzard starts as a result. The six ponies take shelter in a cave, but Puddinghead, Hurricane, and Platinum are eventually frozen in ice. Clover the Clever identifies the cause of the blizzard to be Windigos—winter spirits that feed on hatred.
Despite the animosity between their leaders and tribes, Smart Cookie, Private Pansy, and Clover the Clever maintain warm relations, and they eventually break the windigos' spell through the Fire of Friendship. Their warmth thaws their leaders' bodies and melts their hearts, and the leaders decide to share the new land, naming it Equestria.
Hearth's Warming Eve + Hearth's Warming Day
The day before Hearth's Warming is referred to as Hearth's Warming Eve. It is traditionally celebrated with a pageant that reenacts the tale of Equestria's founding.
The day of Hearth's Warming is typically celebrated by exchanging and opening presents.