I can actually think of some legitimate reasons why individuals might find some days of the week luckier than others. These would be days when the player’s physical condition, and ability to concentrate and make rational decisions is not reduced by prior activities, such as work and/or travel.
Consider my situation, where the closest casino is a 2.25-hour drive away. Suppose I schedule a week-end visit, leaving after work on Friday and returning home Sunday morning. (I cannot remember the last time I did this, but suppose I do.) If I jump into action upon arriving then I will be somewhat fatigued from both the trip and having been at work that day. This could lead to errors in judgment like suboptimal strategy decisions, poor bet selection, and overbetting (to include not quitting when one should). Assuming I do eventually retire and rest up, I should be in better shape on Saturday. Upon preparing to leave on Sunday, however, and assuming I have not tapped out, I may again abandon sound money management and strategy in an attempt to recoup losses or score the big win. Were I to take such weekend junkets routinely I might well consider Saturday, the day when I played my best and therefore did my best, to be my lucky day.