You should have experience with Excel before trying to use formulas to calculate what day of the week the 15th of a given month is, and you have several options to do this.

How to Calculate T-Value From Mean & Standard Deviation in Excel. Calculate t-values in your Microsoft Excel spreadsheet by combining built-in functions with custom formulas. T-values, also known ...

It is difficult to calculate these values manually and while calculators make it easier, Excel is the preferred tool for finding these values over a range of cells.