Microsoft Excel – Averaging for 4 columns

I want to average the hours worked for each of the first 4 weeks in a month over 40 years. Below you can see a picture of my data. For January 1979 I want to average the hours worked for Week 1 to Week 4 of January 1979. So I want to average(AC2:AF2) for January 1979 but for February 1979 I want to average (AG2: AJ2). How can I set up a formula so that Excel always takes these averages without using AD2, AE2 as well.
Picture of the data