You can enter that formula in any cell on the summary sheet and it will sum the exact same cell from every sheet from Jan up to the month specified in A1 without making any changes to the formula.

The way I read your question from the example file is that B3 in the summary should total B3 from the other sheets, C5 should total C5 from the other sheets Z9 should total Z9 from the others, etc.

9 is a parameter in subtotal, 1 = average, 2 = count, 9 = sum (there are more which you can see if you look at the help file for subtotal).

29 is just a multiplier used to convert an array of {1,2,3} up to 12 into months. Neither of these should be changed.