For any given month, all the multiples of 7 fall on the same day. I usually have memorized what day that is for the current month and the next few months. So October is a “[F]riday month” and November is a “Monday month” and December is a “Wednesday month”. You only have to memorize a new one every month.
Now when somebody says, “We should get together one Saturday. Say… December 12.” you can snap back with “That’s a Monday. Do you mean December 10?” All you have to do is find the nearest multiple of 7 and work from there. So when they ask about December 12, that’s close to December 14 - which must be a Wednesday (since December 2011 is a Wednesday month).
Hahahaa, that is a simple handy trick!!!