In Exodus 23:14-17, God says,
14 “Three times a year you shall celebrate a feast to Me. 15 You shall keep the Feast of Unleavened Bread; for seven days you are to eat unleavened bread, as I commanded you, at the appointed time in the month of Abib, for in that month you came out of Egypt. And no one is to appear before Me empty-handed. 16 Also you shall keep the Feast of the Harvest of the first fruits of your labors from what you sow in the field; also the Feast of the Ingathering at the end of the year when you gather in the fruit of your labors from the field. 17 Three times a year all your males shall appear before the Lord God.
"You shall" and "as I commanded you" don't sound like "suggestions" to me. Rather, they sound like laws which is appropriate as they are part of the Law.