[JohnFen]

Yeah, this is what I do. Or used to do. I followed an interesting progression with it -- at some point, I stopped having to actually look at or touch my knuckles to do it, then after a number of years, I found that I (mostly) just have the months lengths memorized.

[ahazred8ta]

It's the odd numbered months 1-7, and even numbered months 8-12. July and August are the two adjacent 31-day months.