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Choose a weekday if you can. Visitor reports consistently describe weekday mornings as the easiest time to enjoy popular attractions, with shorter entrance, show, and greeting lines than on Saturday and Sunday. Arrive before opening if you want first access to popular attractions or better parade viewing positions.
Saturday, Sunday, public holidays, and school-vacation days are much busier, especially from late morning into the afternoon when parade viewing, character greetings, and lunch lines overlap. On busy days, it is more useful to be inside well before the first major show than to arrive mid-day.
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Sanrio Puroland does not have a true weather-driven off-season because it is almost entirely indoors, but crowds increase during Japanese school holidays, Golden Week, Obon, winter vacation, and major seasonal campaigns such as Halloween and Christmas. Rainy weekends and very hot days can also push more visitors indoors.
For a calmer visit, aim for an open weekday outside holiday periods and major event runs. If you want seasonal decorations, special shows, or limited merchandise, those periods can be rewarding, but you should expect heavier queues for parades, greetings, food, and shopping.