Numbers 9:3
In the fourteenth day of this month, at even, ye shall keep it in his appointed season: according to all the rites of it, and according to all the ceremonies thereof, shall ye keep it. In the fourteenth day of this month, at even, ye shall keep it in his appointed season: according to all the rites of it, and according to all the ceremonies thereof, shall ye keep it.
The verse stands where the narrative of Israel's journey resumes with the people encamped at Sinai. It was fulfilled when Israel kept the Passover at Horeb in the first year after leaving Egypt, in the year after the deliverance. It commands Israel to keep the Passover on the fourteenth day of the first month, at even, in its appointed season, according to all its rites and according to all its ceremonies.
The phrase at even means between the two evenings, between the first evening beginning about 3 p.m. and sunset, when the next day began. The Passover belongs in its appointed season in the first month, and the paschal lamb was slain at the close of the fourteenth day, as the fifteenth day began. The feasts God gave to Israel occurred annually and were to be observed only at the time when they were due. The one exception was that those unable to keep the Passover in the first month on account of ceremonial uncleanness were directed to observe it on the same fourteenth day of the second month.
The Passover was observed only once in the wilderness, in the year after the departure from Egypt, with regular observance to begin after Israel entered the promised land. The chapter containing the verse recounts Israel's ceremonies and service and shows the strict order God established for His people so they could learn systematic conduct.