When were those men to keep the Passover rather than on the 14th day of the 1st month?

Those men who were unclean or on a long journey kept the Passover on the 14th day of the 2nd month.

What were some rules for keeping the Passover?

They ate the Passover with bread without yeast and bitter herbs and left none of it until morning while not breaking a bone of the animal.