Formula help deducting over calendar week

I am stumped as to setup a formula to assist with tracking attendance over time, with only giving a value for absences.

Attendees are required to attend 36 sessions, from which classes are held every Monday, Wednesday, and Friday. Persons may begin attending during any class session day.

I am trying to use 2 column values to produce this formula to be displayed in a 3rd column. (see attached example workbook)

The first column is a start date (example: 5/9/11).
The second column will have a numeric value reflecting the total number of absences to date (example: 2)
The third column would have how many sessions they have left to complete out of 36 sessions (36 minus sessions completed plus absences)

Assistance would be greatly appreciated! I hope I have been clear in communicating my needs and problems.