Beha'alotecha - Shlishi
Rain Date
G-d spoke to Moshe in the month of Nisan in the second year of the Exodus. He told him that the Passover sacrifice was to be prepared in its proper time, on the afternoon of 14 Nisan. This was done, as per G-d's instructions. But...
Some people were ritually unclean and unable to offer the sacrifice. They asked Moshe and Aharon what they should do. That was an excellent question, so Moshe took it to G-d.
G-d established a "rain date" for the Passover sacrifice one month later, on the 14th day of the month of Iyar. This extension was only good for people who couldn't offer the korban Pesach on time because they were unclean from corpse contamination or because they were out of town. All the regular rules (matzah and bitter herbs, no leftovers, not breaking a bone, etc.) still applied. However, if anyone not meeting the stated criteria fails to offer the Passover sacrifice on time, he incurs kareis (excision). This rule applies equally to converts and to those born Jewish.