My buyers are selling their house on 39 December and want to also buy my house on 30 December. What could go wrong?
Delays. Delays in their buyer closing. Delays in their buyer's lender funding in time. Delays in their buyer's title agency moving the money around quickly enough. Delays in the banks accepting and releasing their funds quickly enough. None of these should happen, but all of them could.
On top of that, many people take vacation between Christmas and New Years. Will all of the participating vendors be adequately staffed to handle such a tight and important turn around?
Add a clause that penalizes them for delays, or be prepared to close next year. That has the possibility of adversely affecting your tax planning.