Why do they do this? Why not just have a fixed date for these things? Is it so people can use the weekend to celebrate?

If it was a fixed day it would end up being a week day a lot of the time.

Maybe that doesn’t matter, but it’s nice to have on a weekend.

Maybe I’m missing what you mean?

For Mother's Day, possibly, but it's not always the case. Thanksgiving is on the 4th Thursday of November.

Yes

It seems it must be the same in Switzerland where I live, makes sense!