To unlock fun things like a tuxs
A car can still be sold even if its used, and a tux goes through tailoring and all that stuff. And tuxs are rented not as often as cars, especually on special occasions that are a must have like a wedding or prom. Hey im only 17 its just my inference.
Most people dress in bright colors for Cinco de Mayo. Additionally, you may see people wearing ponchos, sombreros and mustaches to celebrate this day.
In this day and age, you can get by with just about anything, but if you don't want to embarrass your date, a sport coat should be considered minimally acceptable attire. Suits are common, and tuxs are not unusual. But, hey…if you enjoy opera and all you have is a pair of Levis…go for it!