The Passover is a marvelous feast to study and learn from. The Passover Lamb is a clear picture of Jesus Christ and His sacrifice for us. Paul said, "For even Christ our passover is sacrificed for us" (1 Corinthians 5:7). Because of the shed blood of Christ, the angel of death will pass over us and we who know Jesus as our Saviour will live forever. It is interesting that many of the traditions of the Jewish Passover--things not stated in scripture--are also typical of Christ and His sacrifice. God was obviously working the Jewish mind even when they did not know it. Also, just as the Passover is a picture of the crucifixion of Christ, the connected Feast of the Firstfruits is a picture of His resurrection.
However, you ask specifically when to celebrate it. First of all, the celebration of the Passover was specifically given to the Jewish people. Its celebration was never commanded to the Gentile believers. There is certainly nothing wrong with a remembrance of the Passover and its significance. We are just not required to keep it as New Testament believers. Yet, if you wish to remember it as the time of the crucifixion and resurrection of Christ, that is fine. In this case, I would suggest you use the Jewish reckoning of the time of the Passover. It will change every year and you will have to consult a calendar. I have often thought that someone should initiate a movement to move away from the present celebration of Easter and its pagan connections by remembering Resurrection Sunday as the first Sunday after the Jewish Passover. Perhaps then we could get away from the eggs and bunnies and remember the true history of the resurrection and its connection with the Passover.
Finally, you ask what real traditions the Lord requires. The truth is, He requires very little if anything in the kind of traditions people expect to have. However, He does not exclude us from all traditions of any kind. Consider the following passages:
Romans 14:5 One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind. 6 He that regardeth the day, regardeth it unto the Lord; and he that regardeth not the day, to the Lord he doth not regard it. He that eateth, eateth to the Lord, for he giveth God thanks; and he that eateth not, to the Lord he eateth not, and giveth God thanks.
Colossians 2:16 Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: 17 Which are a shadow of things to come; but the body is of Christ.
Paul teaches us that special days and traditional celebrations for these days are not a required activity for believers. Yet, he seems to give grace to those who would want to remember certain traditions. However, these traditions are not to be imposed on others and I am certain that these traditions are not to be in opposition to the teachings of scripture elsewhere. For instance, any form of idolatry is wrong--even if it is a long-standing tradition of some church group.