The various ceremonial days, like Pentecost, waving of the sheafs etc, are to mark the times of Gods calendar. Like “not knowing the day nor the hour” is an OT idiom for a certain holiday. You can’t know the day or hour, because the months start with a sighted moon, some years have more months etc. You can’t know the signs of the seasons without knowing the “days of certain events”. That alone is reason to know and memorialize them, so you can be ahead of future events.