Question :
someone has told me that it's forbidden to fast on Saturdays (except Ramadan).Is it true?

Answer :
Praise be to Allaah.
The scholars differed concerning fasting on Saturdays. Some of them said that it is not makrooh at all. Others spoke in more detail and said that if a Saturday is singled out for fasting, then it is makrooh, but if it is joined to the Sunday that follows it or the Friday that comes before it, then it is not makrooh. This is more likely to be the correct answer. And Allaah knows best.

Fataawa Manaar al-Islam, 2/366