Can I use an IVD on an off day to simply boost my pay up, or is it only to drop a trip?

For example, I have a one day trip and then a day off. Can I use 2x IVD's there? One to drop the turn, one to simply add 3:30 of pay?

Finally, do IVD requests have to be in by the 20th, or can they be processed on subsequent PCS runs later in the month?