Careful analysis of the requirements
Doubling and scaling up
A fair dice roll
Some random numbers i pulled out of thin air
Print "$day_of_week months $day_of_month days\n";
Carefully calculating how much paid overtime i need to pay for my next vacation
How much i like/dislike my customer
Results (487 votes), past polls
By rote learning.
Via Genetic memory.
It's provided by my firmware.
I just remember them.
Thirty days hath September
My computer gets it right, usually.
I just ask someone else.
Someone punches me on the first of the month.
Results (103 votes),