The answer depends on the version of Project you're working with.
1. P2000+ server - get your admin to create a calendar specific to your requirements (i.e. Sunday, and Tuesday through Saturday are flagged as Nonworking time).
1a. View | Gantt
1b. Display the column "Task Calendar"
1c. Select the tasks that use that calendar
1d. Change the calendar to the appropriate one.
2. P2000+ standalone -
2a. Tools | change working time | click on "new" and create a calendar specific to your requirements (i.e. Sunday, and Tuesday through Saturday are flagged as Nonworking time).
2b. See 1a thru 1d.
3. P98 - Ah, this is where the challenge lies. P98 supports resource calendars but not task calendars.
3a. Tools | Change working time | click on "new" and create a calander specific to your requirements (i.e. Sunday, and Tuesday through Saturday are flagged as Nonworking time).
3b. (Shhhh -- don't tell anyone, they pay me big bucks to know this stuff.) Click on View | Resource Sheet. Create a resource. Since you only want to do work on these tasks on Mondays name the resource Monday.
3c. Double-click on Monday, find the Working time tab, find the Base Calendar entry and change it to the Monday calendar you created in step 3a.
3d. View | Gantt. Display the appropriate tasks. Assign Monday as the resource.
I wish you had said that you wanted the work to be done on Fridays because then we could have called the resource Friday and, as you know, Robinson Crusoe was the only person to get all his work done by Friday.