So if you need a Passover date, I would recommend those which Ken puts forth. And yes, of course Christians should celebrate the Passover, as Paul even said that we should keep the feast. However Christians should not chastise one another or themselves for little things that cannot be done perfectly in this day and age. We need a remembrance, we need fellowship, but we cannot effect our own salvation.
Colossians 2:16-17 wrote:16 Therefore no one must judge you in food and in drink, or in respect of feast or new month or of the Sabbaths, 17 which are a shadow of future things. Whereas the body is of the Anointed...