Leviticus 23:5-7

23:5 In the first month, on the fourteenth day of the month, at twilight, is a Passover offering to the Lord. 23:6 Then on the fifteenth day of the same month will be the festival of unleavened bread to the Lord; seven days you must eat unleavened bread. 23:7 On the first day there will be a holy assembly for you; you must not do any regular work.