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.

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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.