I already successfully grab the information I need from the calendar, and did so even if I was using only PLAYER_LOGIN or PLAYER_ENTERING_WORLD, so I don't think that's the problem.
According to
this, PLAYER_ENTERING_WORLD fires after ADDON_LOADED and is considered the last startup event. But C_Timer starts ticking down while the loading screen is still visible when you do /reload (again, I think this is a bug), so it doesn't work properly with alert frames.
Is there not an event that fires when the game is actually playable, player in the world? Maybe one that actually signifies something besides "game now playable" but will reliably occur at that point? It could be one that triggers many times, even, just so long as the
first time it triggers after PLAYER_ENTERING_WORLD is when the game world is shown.