at that time
Exodus 12:6
And ye shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening.
Exodus 12:18
In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even.
because
2 Chronicles 29:34
But the priests were too few, so that they could not flay all the burnt offerings: wherefore their brethren the Levites did help them, till the work was ended, and until the other priests had sanctified themselves: for the Levites were more upright in heart to sanctify themselves than the priests.