On the fourteenth day of this month, at twilight, you shall keep it at its appointed time. According to all its rites and ceremonies you shall keep it."
Numbers 9:3
Surrounding context
9:2 "Let the children of Israel keep the Passover at its appointed time.
9:3 On the fourteenth day of this month, at twilight, you shall keep it at its appointed time. According to all its rites and ceremonies you shall keep it."
9:4 So Moses told the children of Israel that they should keep the Passover.