We already quasi-have this to a small degree with World of Warcraft's heroes. Some quests during the Harvest Festival over on the Horde side of things, for instance, have us visit Grom Hellscream's monument and pay tribute to his sacrifice in removing the orcish blood curse. I think we could have two faction-specific holidays that show off the Warcraft history without being spiritual dopplegangers to real life events.
For the Alliance, a holiday could be created to honor the sacrifices of the men and women who charged through the Dark Portal and destroyed the other side, leaving them stranded on Draenor for what appeared to be forever. Nethergarde Keep could be the site of the holiday, as Alliance players go to pay their respects at the site of one of the bravest acts of heroism the Alliance exhibited in the war against the Legion-dominated Horde.
For the Horde, players could partake in some sort of release from bondage day, since at some point all of the races that currently make up the Horde were enslaved either physically; through addiction, like the blood elves; or through conflict, like the tauren. Rather than be a holiday wishing for the destruction of slavers, this time would be of appreciating the relative freedom that the races of the Horde now enjoy.
What do you think some in-game holiday, unique to the game world and lore, be? Would you like to see Blizzard make up actual in-game holidays instead of following by our own cultural moments?