Given a date, how to find if it's the second or fourth Saturday of the month

Question

In my Rails application, I need to find if a given date is the second or fourth Saturday of the month. What's the efficient way to do this? Is there a gem I can use?

Answer 1

• Days 1 to 7 are week 0
• Days 8 to 14 are week 1
• Days 15 to 21 are week 2
• Days 22 to 28 are week 3

To get the week id, we can calculate (date.day-1)/7. Since the id is zero-based, the second and fourth saturdays have an odd week id :

def second_or_fourth_saturday?(date)
  date.saturday? && ((date.day - 1) / 7).odd?
end

Answer 2

The second Saturday has to be in the day range 8-14, and the fourth in the day range 22-28. So I think this should work

def second_or_forth_saturday?(date)
  return false unless date.saturday?
  (8..14).include?(date.day) || (22..28).include?(date.day)
end