Numbers 9:11
9:11 They may observe it on the fourteenth day of the second month 1 at twilight; they are to eat it with bread made without yeast and with bitter herbs.
Numbers 28:17
28:17 And on the fifteenth day of this month is the festival. For seven days bread made without yeast must be eaten.
Numbers 29:2
29:2 You must offer a burnt offering as a sweet aroma to the Lord: one young bull, one ram, and seven lambs one year old without blemish.
1 sn The delay of four weeks for such people would have permitted enough time for them to return from their journey, or to recover from any short termed defilement such as is mentioned here. Apart from this provision, the Passover was to be kept precisely at the proper time.