I don't know how things work in other countries, but most of the people I know here in the U.S. who earn a paycheck (not freelancers) get paid on Fridays.
If your special promotion does not run over a weekend or two, you're losing out on sales. Consider starting your promotion mid week (wednesday or thursday) and ending it on a monday or tuesday. Run it over at least two weekends if you can.
I can't count the number of times a vendor ran a special one day promotion or a "3-day sale" a day or 3 before payday and I missed out. And being disgruntled, I never bothered to get it at full price because I felt cheated.
That's where you get the expression: "The Eagle flies on Friday, Saturday I go out to play". But I don't think that's the pay standard any more except maybe for cash businesses.
I get paid twice a month and it is by EFT direct-deposit to my bank accounts, so my "eagle" flies on different days each month as the calendar advances around the year. It's nice not having to drive to the bank to cash/deposit a check, huge time saver.
As far as timing promotions, yeah, you want to give the customer time to take advantage convenient to their schedule. But I see this taken to silly extremes when a Memorial Day or President's Day sale runs all month.