Way back in Mitzvah #230, we had a prohibition forbidding us from not paying our employees promptly. Here we have a positive mitzvah obligating us to pay workers on the day wages are due. This applies not only to money owed for services rendered but also to fees for rentals.
The reason for this mitzvah is obvious: people rely on their wages. Not to pay on time creates hardship and crushes the worker's spirit. The employee did his share, now it's time for the boss to do his.
One only violates this mitzvah if the employee asks to be paid and the employer flat out refuses. If the employee doesn't ask or if the employer does not have the funds available, he is not in violation. (Of course, every employer should strive to have cash available when payday rolls around.) If payday falls on Shabbos, it is pushed off until Sunday and it's not the fault of the boss.
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in tractate Baba Metzia (110b-111b) and codified in the Shulchan Aruch in Choshen Mishpat 339. This mitzvah is #200 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #66 of the 77 positive mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.