Adding to the other comments, the best way to use MS Project is NOT to try
to input task schedules yourself, whether you're trying to tell it to start
the task on the weekend or some other time. Project's job is not to parrot
back at you the dates you have determined you want tasks to occur. You
don't tell Project when you plan to do the tasks, it's job is to tell YOU
when you should be planning to do them.
A far better way than what I think you're doing is to tell Project
generically when tasks can take place (the calendar found in Tools,
ChangeWorkingTime and setting it to be the Project Calendar), what tasks
need to take place, the expended length of time each will take to complete,
and the relationships expressed through the links. The Start and Finish
fields are *calculated* fields and Project's calculation of them tell you
when they can be scheduled.