Jamieson, Fausset and Brown on John 12:12

12. On the next day--the Lord's day, or Sunday (see on Joh 12:1); the tenth day of the Jewish month Nisan, on which the paschal lamb was set apart to be "kept up until the fourteenth day of the same month, when the whole assembly of the congregation of Israel were to kill it in the evening" (Ex 12:3, 6). Even so, from the day of this solemn entry into Jerusalem, "Christ our Passover" was virtually set apart to be "sacrificed for us" (1Co 5:7).