On september you can see a lot of mexican flags vendors, little or big, hats, bugles or anything you will use next 15th septiember night, at 11:00 PM when all mexican people celebrates their independence day. The celebration ocurrs in each central square all over the country, so I think you cant lost the event!!!.
September 16 is a national party, is our independence day, but party begins a night before, september 15, where president and other governors each one in his own state, ring a big bell to remember the night when priest Miguel Hidalgo ring the bell to call every body in a little town to fight agains bad govenment.
During september mexicans celebrate the Independence month, so you can enjoy a lot of national flags all over the entire country