Most electric providers determine your rate per KWH based on your peak usage for the month.
If they read the meter on 11/21, and you started blowing on 11/20, then the entire previous month is billed at a much higher rate.
This is why, traditionally, everyone stops blowing around the same time, the end of Presidents week. If you blow a single day after the meter is read, the whole month of march is much more expensive.
Not exactly sure why it is this way, but I have heard this from Mike Pratt, Laz and many other GMs.