3 months ago I started working in Switzerland, and my Boss just asked me if I wanted an advance for the December salary or if I wait for January. My question is if I get the Money in January if it will Count for next year's taxes or this year? I want to save on taxes, and this year (jan-dec) I didn't earn that much so it's suppossed that the taxes will be less, right?

It depends. I certainly got money this year that went on last year's taxes, as that's what was written on my salary certificate.

No, they are pro-rated, so the rate for the three months you worked will be the same as if you had made the same salary all year.

Tom

Normally, I get my pay at the beginning of the next month. So December's pay, comes at the beginning of January... but it is still December's pay and is included in that tax year. Your boss is just offering to pay you earlier as it's Christmas and maybe you need the money. The money will be taxed in 2015 tax either way,