Friday Be An Unlucky Day
One day a kid challenges Riddle master Gogo to solve a riddle. The child claimed that he was smarter than Gogo.
Gogo thought to himself," I can calculate out how many days there will be for the 13th on each month if i count from the beginning of the year (January 1). Then i can divide total days by 7 to get the remainders. I will also need to consider the leap year. Through the whole year i had all kinds of remainders, from 0 to 6. The minimum of occurrence for all the unique remainders was 1. It means that there is at least one Friday on 13th. In a regular year, the best chance you can get 3 Fridays on 13th, which are in February, March and December because the remainders of these 3 months are 2. In a leap year, the best chance you also can get 3 Fridays on 13th, which are in January, April and July because the remainders of these 3 months are 6.