If you add a sheet, call it say, Last, and then when you add new weekly sheets make sure it's between Last and the the first sheet you're summing, your formula can then remain =SUM('01_01:Last'!I12)
Make sure you have nothing on sheet Last in cell I12



This difficulty stems from the source data for your summary sheet all over the place.
I'd suggest you put your games results in a single table and include a date column. You'll then easily be able to get a whole host of summary details from that one table, and you won't have to add new sheets.


Let's have a peep of one of your weekly sheets, or better, attach a workbook here; I'll rearrange the data and give you a new Master/Summary sheet. Do it sooner rather than later because I won't be near a machine for a couple of weeks.