This year (by June 2010) IT already happened thrice and will occur one more time, on October 1st.

Next time SAME THING will occur on two different dates in 2011 and on 6 different dates in 2012.

In 2013 - only during one day and then in the following years IT will present itself at least once a year till 2036.

In 2037 - not even once.

What is IT (a.k.a. SAME THING )?
If IT = f(YEAR) what is f(2020)?
In what year in this century f(YEAR) is maximal?