Numbers 9:11-12

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. 9:12 They must not leave any of it until morning, nor break any of its bones; they must observe it in accordance with every statute of the Passover.

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.