This took me way longer than I would have preferred to write.

day_of_the_week_in_month.rb

require 'active_support/time'

def Time.month; self.strftime("%_m").to_i; end
def Date.month; self.strftime("%_m").to_i; end

def fridays_in_month(date_or_time)
  month        = date_or_time.month
  current_week = Date.new(date_or_time.year, date_or_time.month).
                      prev_week(day = :friday).
                      next_week(day = :friday)
  fridays      = []

  loop do
    fridays << current_week if current_week.month == month
    current_week = current_week.next_week(day = :friday)
    break unless current_week < date_or_time.end_of_month.to_date
  end

  fridays
end