SUM on varying number of cells in VBA

I am writing sum of few cells from the row above, to a single cell and then merging the cells
I need to achieve this in vba but the problem is that the number of columns I need to use in the SUM formula can change month by month, though I know how many columns are there in each month (See attached excel) and I am using A1 style to write formula in my code. I couldn't find any way to write formula in vba in such a way that you can change column name to variable (so that first sum covers C2 - C6, second covers C7 - C10 etc.).
Any help will be highly appreciated.