Find the next available date from a list of dates

[rplohocky]

I built a form for about 25 people to use. Once a certain process is complete in a record, the user must find the record and then select a date from a list of available dates. Each day there would be 5 open slots, only Mon, Tues, Wed, and Thurs. I am looking for a way to create a field that the user would chose the next available date and insert that date in the field. Once that date is selected it would not be available for any future changes on any of the other records as well.

Thanks.