In Portland (there are other midweek series in Eugene, Bend, and Hood River Oregon):
Monday - Master's (30+) and juniors at PIR (Portland International Raceway) - sort of a crit on a 1.9 mile auto race track with no real corners. They have a point format with several hot spot laps with 3/2/1 points awarded for 1st-3rd place and points for the top 10 at the finish. Also a weekly Short track MTB series also at PIR.
Tuesday - same format PIR races as Monday, but not divided by age. I like all the PIR races (both Mon and Tues) because I'm a 3 and I can race P/1/2/3 races and get some experience against the fast guys - although the most of the fast guys train through these races.
Wednesday - Mt. Tabor - 1.2 mile loop with 130' hill - you are either going up or down. Painful race. Crit series starts up when the Mt. Tabor series ends in July. Also track development class.
Thursday- Track races - different races each week of all types.
Friday- track sprint races
Also, I think there are Kids races a few days a week on the track and at PIR as well.