_ _ _ _ _ _ _
If it rains on Sunday, Monday, Tuesday, Wednesday, and Friday you write:
X X X X X _ _
There are different ways to put five X's.
The probability that it does not rain on the last two days (I assume that is what you mean by 'weekend') is getting the first five X's and the last two empty. There is only one way to do that. Therefore, the probability that it would not rain is . Thus, the probability that it would rain is .