My period is on 31th day of the month and we have sex on the 7th day of the month is this possible that it's a boy?

It's either a boy or a girl. Having sex on certain dates during specific times of your cycle doesn't guarantee one sex or the other.

A boy or girl is not by days of the month or days of your cycle. So it will be either a boy or a girl.

So to answer your question, yes there is a chance it is a boy.