How can I work out what day of the week it'll be after x amount of days?
Some kind of formula to this......
As was suggested number the days. $\displaystyle \boxed{\begin{array}{lllllll} {Su} & M & T & W & {Tr} & F & S \\\hline 0 & 1 & 2 & 3 & 4 & 5 & 6 \\ \end{array} }$
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.