The Harvest Festival is largely based on the Eastern/Asian tradition of the Mid-Autumn Festival-- I'd say it is largely a Chinese thing, mostly because that's my experience with it, having grown up celebrating the holiday; but I don't want to mistakenly exclude anyone. In the United States, it's better known as the Moon Festival or the Mooncake Festival.

The main story goes like this: Around the time of mid-autumn, harvest is in full swing and, if things are good, there's a lot of crops to harvest. But sometimes, there isn't enough daylight to finish the job. Luckily, the full moon around this time happens to be really big and really bright-- so bright, in fact, that farmers are able to work through the night if they need to, just using the light of the moon.

Over time, non-agrarian families now have the tradition of getting together for a large meal, honoring their ancestors by visiting their graves and leaving food, and eating delicious mooncake while observing the moon and having a general grand ol' time. (Mooncakes have their own legend associated with the festival, btw, in addition to many other legends and myths.)

You may be asking, "What, mid-autumn?? How is it even mid-autumn yet when the autumnal equinox hasn't even gone by?" The Chinese (like many Asian cultures) follow the lunar calendar. To them, it's Mid-Autumn. If you've been playing WoW for a while, you probably notice that of all the world holidays celebrated in the game, only two of them tend to move around on the calendar without a fixed date: this one and the Lunar Festival (which is based on Lunar New Year).

Pilgrim's Bounty is based on the US holiday of Thanksgiving... paying homage to the Puritan pilgrims who came to the continental US on the Mayflower, plus all of the turkey references.