If we divide 25 by 7 we get a remainder of 4.
If today is Sunday then in 25 days it will be Thursday: 0+4=4. Four is Thursday.
But if today is Friday in 25 days it will Tuesday: 5+4 divided by 7 has a remainder of 2 and 2 is Tuesday.
So divide x by 7 to get a remainder. Add that to the number of the day and then find the remainder when divided by 7 that number day in x days later.