My kids are always amazed when I do tip calculation or other every day math in my head faster than they can on a phone calculator or paper.
Can you count by 2s? Is it as easy as 1-2-3? Do you like your 9 to 5 job at the 7/11?
Consider these dates in 2015 and into 2016.
2/last, 3/21, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, 12/12 and 1/23 (the following year)
The dates above will all fall on a Saturday. The same series for 2016 into 2017 will fall on a Monday
Leap year complicates things so we have to start with February last, either the 28th or the 29th, but if you verify one of those dates and can remember the series, you can always figure out what day of the week any day in the year falls on.