My parents are doing a mutual exchange (housing association) and are going in to sign contracts on the Friday in a few weeks time. Does it tend to mean they then move the weekend after signing with a tenancy to start the following monday?
The terms of exchange, including the date of vacation of your parents' current property will be stipulated in the contract.
This is what they are going in to sign. They should ask to see a copy or draft beforehand, so that they will have had a chance to read through what they will be signing, and to let them know the time frame to which they will be working once the contract is signed.