The New York Times crosswords are easiest on Mondays, and they get more difficult as the week progresses. Friday and Saturday are generally too difficult for new solvers, and quite a challenge for even more experienced crossword solvers.
When discussing a puzzle’s difficulty, it’s not uncommon for regulars to say, “It was about a Wednesday level puzzle …” for example. The day of the week has become a unit of measure (for difficulty) within the crossword community.
Of course, the Sunday puzzle doesn’t follow the weekday “grading” system. It’s much larger than the daily puzzles, but the difficulty level isn’t usually much greater than a Wednesday or Thursday puzzle.
And Friday/Saturday puzzles have something else in common with each other (in addition to being more challenging): they don’t have a theme. They are “themeless.” Not having a theme doesn’t make them more difficult per se, but it removes a kind of inside “hint” that you might otherwise use as a crutch when solving. (And it means that you don’t experience an “Aha!” moment when you discover what the theme entries have in common.)
Fridays and Saturdays — at least in the New York Times — are really for solvers who love the challenge of a tough crossword puzzle.