From https://valheim.fandom.com/wiki/Events:

"Every 46 minutes there is a 20% chance for a random event;

Available events are based on which enemies have been defeated in the game world.
Most events require defeating a boss to become available (see Enabled by column)
Some events are permanently disabled after defeating a boss (see Disabled by column)
For each event, check that any of the players fulfills the following conditions:
The player is in the required biome (see Biome column).
The player is not in a dungeon.
The player is within 40 meters of at least 3 base structures. This is not required for the "You are being hunted..." event."

Originally posted by Cadillac:

I've been playing for several days now. killed two bosses but still no raid events appear, only some isolated monsters but no raid event.

any ideas?

According to what people have discovered is you need to be in render range of a workbench which is part of 3 or 4 structures. When the hidden timer goes to trigger it will start. If you're out in the woods hunting, mining, etc it won't trigger. I went weeks without an invasion. Then I build a workbench, some simple walls and a chest to dump all my ore halfway to my base. Minutes later a wolf army attacked.

It's random. On one seed I could time the hour by the raids I got but then I was spending a LOT of time building whilst other players on the server were adventuring. On another seed, hardly anything, but then I was out and about a bit more.

Originally posted by knighttemplar1960:

The game only checks every so often and if you aren't in a "base" at the time it should come up nothing happens. You'll experience more raids when you start farming crops, tending livestock, and smelting ore. Activities that keep you in the base for longer stretches of time.

and then you'll be like the rest of us saying "f--- another raid!"

I spent about a week of game time doing base improvements in the Black Forest. Had three Troll raids and two other independent trolls (1 and 2 star) during that time. That's RNG for you.

Good luck? Or maybe bad.
I just started a new game, had three Forrest events so far, two at my base and one at a copper mine I was working. So I'm getting them......

The game only checks every so often and if you aren't in a "base" at the time it should come up nothing happens. You'll experience more raids when you start farming crops, tending livestock, and smelting ore. Activities that keep you in the base for longer stretches of time.