I need to calculate the start date of a service based on the day of the week,
i.e. Monday, Tuesday, etc.

For example:
All of our clients are paid weekly. Their paydays vary between Monday and
Thursday. Lets say client "A" starts a service on a Wednesday (12 July
2006), but he/she is paid on a Monday, we would take the start date of their
service as the following Monday (17 July 2006). In other words, we can only
pay clients weekly, starting from their very next payday.

I need to enter a formula to work out the 17th July in the example above.