Answer:
\boxed{5}.
Explanation:
If there were a total of 20 football games in the season and 6 were played at night, then 20 - 6 = <<20-6=14>>14 games were played during the day.
Since the season is played for 4 months and each month has the same number of games, then 20 / 4 = <<20/4=5>>5 games were played each month. Answer: \boxed{5}.