Passover and the Crucifixion
Matthew 26:17-19; Mark 14:12; Luke 22:7-15; John 18:28; 19:14, 31; 1 Corinthians 5:7
The central events of the Christian faith are placed by all four Gospels inside the Passover festival. The dating is not incidental scenery; the apostolic writings make theological use of it.
The setting
The Synoptics present the Last Supper as a Passover meal (Mark 14:12-16). John's account has the Jewish leaders avoiding defilement so they may eat the Passover on the morning of the trial (John 18:28) and dates the condemnation to 'the preparation of the passover' (19:14).
The apostolic reading
'For even Christ our passover is sacrificed for us' (1 Corinthians 5:7). Paul reasons directly from the feast to the event, and the resurrection on 'the first day of the week' (Mark 16:2) is what gives the Christian Sunday its meaning.
The chronological question
Whether John and the Synoptics differ on the exact day, or whether they can be harmonised (different reckonings of when the day begins, different Galilean and Judean customs, or John's 'preparation of the Passover' meaning the Friday of Passover week), is a genuine and long-standing exegetical dispute. It is not settled here and it does not need to be for the point that a date matters.
Why this drives everything downstream
Because Pascha is tied to Passover, and Passover is lunar, the Christian Pascha could never simply be a fixed solar date. Every calendar controversy in this quest descends from that single fact.