I don't think there's a better time to play Slots in term of days, nights, weekdays or weekends.
A machine pays after it had swallowed a certain amount, so you have to be lucky and sit at one on which a lot of people had lost. When? Who knows. It could be anytime of the day.
Have you heard of some pros who observe a certain machine for hours and hours, sometimes for days before they get on it? They only play when they know that that particular machine is about to pay off some interesting prizes, maybe even a jackpot.