Holy Saturday is almost a non-event in the cycle of Holy Week. It is the day that Jesus lay dead in the tomb. He was busy, of course, decending into hell to gather all the good souls who died before he did (Abraham, Moses, etc.--you didn't really think God was just going to leave them there, did you?). However, all the potential witnesses were off observing the Sabbath, so there's really no information in the Bible about what was going on that day.
However, on Holy Saturday, in the evening, we celebrate the Easter Vigil. Properly, it should end just after midnight so that you are actually celebrating Easter on Easter day (though, as Kristen pointed out, if you are Orthodox, you're there until dawn--that's dedication for you!). However, it depends on the parish whether or not you make it to midnight (it's sort of like "midnight" Mass on Christmas Eve; it does seem ridiculous to call the church to ask what time midnight Mass is, but in some churches, it starts at 9pm and only lasts an hour, and therefore isn't really at midnight). I just got back from Vigil Mass, so obviously we didn't make it to midnight. However, it is Easter for us now, no matter what the clock says (which is good because now I can have cookies and candy)!
The Vigil is pretty amazing, as far as Masses go. We start off with a fire and if the church is big enough, they'll actually have it in church. Tonight, we had it outside. Then, the Easter candle is lit. This candle will be lit during the entire season of Easter--yes Easter is a season, it lasts 50 days--and at baptisms and all baptismal candles will be lit from it. Then we all light little candles and process into the church and the priest chants "Christ our Light," and we respond, "Thanks be to God," and we all take our seats and blow our candles out and sit in darkness during 7 Old Testament readings which take us through the history of the Israelites and through some of the prophesies and 6 psalms--one between each reading. Then, we get to sing the Gloria! We have not said the Gloria prayer through all of Lent, and now the lights come up and we belt it out, Glory to God in the Highest and peace to His people on Earth! Then, there is a reading from one of the letters of Paul and then we sing the Alleluia (which we also haven't said all during Lent) and we have the Gospel reading. The priest gives the homily, then we get to the really special event--the initiation of the catechumens.
In the very early days of the Church, only adults entered the Church and only after a long period of study. They received the sacraments of initiation--Baptism, Confirmation, and Eucharist--at the Easter Vigil Mass. This tradition is upheld by the adults who enter the Church today. We had several baptisms, one guy who was already baptized and received first Eucharist, and a bunch of confirmations. It's so wonderful to see people come to the Church of their own free will. I have heard enough anti-Catholic sentiment that I am always amazed when someone who wasn't born into the Church joins the Church.
After the sacraments of Baptism and Confirmation, we have the Liturgy of the Eucharist--just like we usually do--and the newly baptized and confirmed receive their first Eucharist and then the rest of us come up for communion. Then, there's a final prayer and the closing hymn, which inevitably is "Jesus Christ is Risen Today."
Easter, the commercial holiday, probably has more pagan symbolism than Christmas. The bunnies, the eggs--symbols of fertility. And, of course, the holiday itself gets its name from a pagan holiday. But, Easter is the soul of Christianity. As it says in one of my favorite books, A Prayer for Owen Meany, by John Irving, "Anyone can be sentimental about the Nativity; any fool can feel like a Christian at Christmas. But Easter is the main event; if you don't believe in the resurrection, you're not a believer." And it's true. It's easy to believe in a baby being born. It's unfathomable to imagine the kind of love that could cause someone to allow himself to be tortured for hours and hours, then executed, but it's unfortunately easy to believe that someone can be tortured in that manner. But to believe that someone came back to life after death.... And it was a real death. A true death. His body was no longer alive and His soul was separated from his body (making that little trip to hell). That is the Mystery of Easter. And if you can't believe that, then you can't call yourself a Christian.