They are both correct. The first one is better is you simply want to discuss a completed past event: that you played football yesterday. The second one is better is you want to talk about something that happened DURING THE TIME that you were playing football. Example: "Yesterday, I was playing football when, suddenly, it began to snow." Or, "Yesterday, I was playing football when I realized that I really don't like the game very much." Another way to use the second example is to contrast the way you spent your time in the past with the way you spend your time today. Example: "In past years, I was playing football or going to parties everyday. Now, I am devoting my life to helping others."

Both are correctly, although sentence 1 would be used in common English . Sentence 2 is just more proper.