Wrapping my mind around the Passover

A friend was asking about the Last Supper - and if it's the Passover Seder - or if it's before the Passover Seder. It certainly is the same pattern as the Passover Seder I've done before, but if we look at the timing, there's a question about the slaughter of the Lamb - If Yeshua is the Lamb, then he must be slain on the 14th of Nissan in order to be the Passover Lamb - which we know he is. According to the timing, Yeshua was put on the cross at 9 am and died at 3pm - this must have been on the 14th of Nissan. This means the Lord and the disciples had a seder supper on the eve of the 14th of Nissan. This is the day of preparation for the Passover Sabbath. The following day would have been the regular Sabbath day, and then the Lord was ressurected on the following morning. This means, he was dead and in the grave on the 14th of Nissan, the 15th of Nissan, the 16th of Nissan and ressurected the morning of the 17th of Nissan.

So my question about the seder still hasn't been answered, but the mystery of Yeshua in the grave for 3 days has been solved.