The Passover was an important celebration of God’s relationship with His people: it was so important, God has said celebrating it would be like a mark on their hand or forehead.
A year after leaving Egypt, as Moses put up the Tent of Meeting and as the Levites were being consecrated, the people were commanded to celebrate the Passover in the wilderness for the first time. So they did:
Numbers 9:4-5 So Moses told the people to celebrate the Passover in the wilderness of Sinai as twilight fell on the fourteenth day of the month. And they celebrated the festival there, just as the Lord had commanded Moses.
But some hadn’t prepared properly. Through no fault of their own, they’d found themselves ceremonially not ready.
This was something Moses couldn’t make up an answer to. The Passover was too important for that, so he told them:
Numbers 9:8 Wait here until I have received instructions for you from the Lord.
The answer? Do it next month.