New King James Version (NKJV)
Numbers 8
22 After that the Levites went in to do their work in the tabernacle of meeting before Aaron and his sons; as the Lord commanded Moses concerning the Levites, so they did to them.
23 Then the Lord spoke to Moses, saying,
24 "This is what pertains to the Levites: From twenty-five years old and above one may enter to perform service in the work of the tabernacle of meeting;
25 and at the age of fifty years they must cease performing this work, and shall work no more.
26 They may minister with their brethren in the tabernacle of meeting, to attend to needs, but they themselves shall do no work. Thus you shall do to the Levites regarding their duties."
Numbers 9
1 Now the Lord spoke to Moses in the Wilderness of Sinai, in the first month of the second year after they had come out of the land of Egypt, saying:
2 "Let the children of Israel keep the Passover at its appointed time.
3 On the fourteenth day of this month, at twilight, you shall keep it at its appointed time. According to all its rites and ceremonies you shall keep it."
4 So Moses told the children of Israel that they should keep the Passover.
5 And they kept the Passover on the fourteenth day of the first month, at twilight, in the Wilderness of Sinai; according to all that the Lord commanded Moses, so the children of Israel did.
6 Now there were certain men who were defiled by a human corpse, so that they could not keep the Passover on that day; and they came before Moses and Aaron that day.