Hi Madi,

I won't give you an equation or a formula but I'll show you how to reason out the solution.

First of all July 4, 2003 was on a Friday.

Step 1:

There are $365$ days in a year and $\frac{365}{7} = 52$ with a remainder of $1.$ Thus there are $52$ weeks in a year but the remainder of $1$ means that the calendar gets shifted $1$ day. Thus it looks like July 4, 2004 was a Saturday.

Step 2:

Did your time span include February of a leap year? 2004 was a leap year so yes, your time span did include February of a leap year. Thus at the end of February, 2004 the calendar got shifted one more day and July 4, 2004 was a Sunday.

I hope this helps,
Penny