Do your longest possible run. No km target just ensure you reach the 50km weekly target and more.
Saturday runs are best done in the mornings with a target of being done by 09:00-10:00.
best advise is to run to your closest run event the Saturday(Official race or local Parkrun).
Long runs on a treadmill are not advisable. Rather ride a stationery bike or do other forms of activity that will compensate the calories to be burnt.
This is the best day for a diet cheat.