If you don't want to wait that long, you can open the console and set the game to a sort of creative mode which will allow you to build / craft anything "free" using no materials.

The Maypole (as well as all other seasonal items) become available that way.

If you don't want to "cheat" the items in, you could go to the site which shows you Valheim maps by typing in the world seed, and look for a map which has a naturally occurring maypole. I'm sure someone can link the site as I don't know it off hand)

Of course, the non-cheat way would be to run around and hope you get lucky enough to find a maypole on the map you are on or create a different map(s).

In 500 hours I have only ever found 2 worlds that had maypoles on them, and both times they were in crappy locations I didnt want to build in.