It is marks based rank test. Although you are ranked on marks, you will not know your marks after the results are out. I gave PESSAT on 6-July 22,12:30pm which was a Wednesday . There were about 300+ other students who wrote PESSAT same day as me.

I fill the slot of writing exam on a weekend or weekday considering that weekday less number of people will be writing and there will be a chance that my percentile will go up, so does writing on a weekday or a particular day will help me get good marks/rank in PESSAT, any advice from seniors who have written PESSAT?

I believe it doesn't really matter when you take the exam, all that matters is how much your score and the difficulty of the paper in your shift.