As I write to you, it's about 7:30 PM on Monday night. The significance of this is that's just about sundown meaning Passover has just begun. Passover always falls at this time of year, moving around slightly on our calendar due to the fact the Jewish calendar uses a lunar schedule as opposed to our solar schedule. See more on this following the description.
Passover is one of the highest days of holiness in the Jewish year. It was the one day of the year when the Jewish High Priest would enter the Holy of Holies where the Ark of the Covenant was placed. He would pour out blood from the spotless lamb onto the mercy seat (the top of the Ark) and thereby atone for the sins of the people from the past year. It was followed by a feast and it was a celebration that was one of the highlights akin to our Christmas celebrations. It's celebrated beginning at sundown tonight until sundown tomorrow (Tuesday).
So here's why this matters to us. When Christ came to Jerusalem (as described in Scripture - Matthew 21:1-17), he came the week of Passover. When Christ gave himself on the day we call Good Friday, he did so on the day of Passover. He himself was the spotless lamb and he himself was the sacrifice made to atone for the sins of all the people. Each step of the path Jesus took during Passion Week was inextricably tied to Scripture foretelling it. I hear you saying "so what?" The big deal is that it ties the Old Testament and the New Testament together - the fancy phrase is "consistent hermeneutic" - meaning the message between the Old Testament and the New Testament is the same. Now THAT'S a big deal! Happy Passover!