Exegesis:

ēlthen … hē hēmera tōn azumōn ‘the day of the (feast of the) unleavened bread arrived.’ ēlthen contrasts with ēggizen ‘was drawing near’ in v. 1. For ta azuma cf. references on v. 1.

[en] hē edei thuesthai to pascha ‘on which the Passover lamb had to be slaughtered.’ For thuō cf. on 15.23. edei denotes religious compulsion.

Translation:

On which, or starting a new sentence, ‘this is (the day) when.’

Lamb, see on 10.3.

Sacrificed. Though the slaughtering of the passover lambs took place in the temple it was not a sacrifice to God. Accordingly a verb for (ritual) slaughtering is best used, or simply, ‘to kill.’

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.