1. What is it?
A mix of spices used in autumn harvest celebration foods, including pumpkin pie, apple pie, apple pastry, apple crisp, squash pie, pear crisp, and things that are supposed to taste like them, for example, spiced cider, spiced hard cider, spiced coffee, spiced wine, spiced mead, and spiced ice cream.
The basic spices are:
So, for those of you outside the USA, it's basically the same spices as Lebkuchengewürz except without the coriander and star anise. When one sees a Facebook meme mocking pumpkin spice that starts with "white girls be like" they are referring to the fact that this holiday spice mixture is similar to a German holiday spice mixture. The idea behind those memes is that only Germanic descended people go nuts over this flavor, but that's really not true. All kinds of Americans like pumpkin pie and apple pie.
2. Why is it a seasonal flavor?
It's used to flavor things made from seasonal produce like pumpkins and apples. The harvest seasons for pumpkins, squashes, apples, pears, and so forth in the USA extend from August to December, since different vegetables and fruits come on at different times and the USA is so large that it has many different climates with different dates of the onset of frost.
3. Why are people mocking it?
Americans have been conditioned to time-police our holidays by observing the practice of our large corporations to start selling holiday related merchandise while another holiday is still coming up (for example, putting out Christmas decorations before Halloween), and in the case of Christmas, the practice of piping holiday music into the stores starting so early that it is nearly universally acknowledged that it reaches homicidal levels of annoying by the time the actual holiday rolls around. Americans think September is too early to start selling harvest celebration flavors. The people are attempting to time-police our corporations over it and shame each other into not supporting the practice by purchasing the product too early. The purpose of this social shaming is to cause the public to wait to make holiday purchases until the correct holiday season, and thus to cause market pressure to influence corporations to wait to attempt to sell holiday products until the correct holiday season.
4. Why are people saying it's "too early"?
To understand why Americans think selling a harvest celebration flavor almost precisely on the date of the autumnal equinox is "too early" one must first realize that the USA has an official national holiday to celebrate harvest in November, Thanksgiving. It's not celebrated in other countries at all, but it's actually our biggest national holiday -- at least for adults. Kids get a week off for Christmas and Easter, but adults only get 1 day for those if they work at a place that closes for national holidays, or if they work at a place where they can request religious holidays off and have requested the Christian set of holidays, but we get two days for Thanksgiving, a Thursday and a Friday, making for a 4 day weekend for those who get weekends off. It's the only national holiday that's more than one day.
5. Where can I learn more?
To read more about the origin, functions, importance, and modern practices surrounding Thanksgiving, and other holidays celebrated in the USA, see my book American Celebration.