Passover - When Is Passover to be Celebrated?

Passover Puzzle - When Should it Be Celebrated? 

Passover, the feast in remembrance of God rescuing the Israelites from Egypt, is  celebrated by Jews and Christians. It is the last meal Christ had with His disciples. When is Passover and the 7- day Feast of the Unleavened Bread that follows it?
What do the scriptures say?

Scriptures About Passover

In  Exodus: 34. 18.  Moses is first  commanded :

Celebrate the Festival of Unleavened Bread. For seven days eat bread made without yeast, as I commanded you. Do this at the appointed time in the month of Aviv, for in that month you came out of Egypt.

  So when is the month of Aviv?
In Leviticus: 23. 5. -6  We find:

The Lord's Passover begins at twilight on the fourteenth day of the first month.  On the fifteenth day of that month the Lord's Festival of Unleavened Bread begins; for seven days you must eat bread made without yeast. - Bible Offline

Throughout the first Testament over and over we find this reference to the 14th day of the first month.
Numbers 9:10-11 does make an exception for those who were ceremonially unclean to celebrate Passover beginning on the 14th day of the second month.
 Again in Deuteronomy, celebrating Passover in the month of Aviv is commanded along with the specification that no yeast should be found in the land during the seven day  feast of Unleavened bread in and that none of the meat from the first night should remain until morning.
Some more references to the 14th day of the first month are found in;

• Leviticus: 23. 5,  
• Numbers: 9. 2 -3, 
• Deuteronomy: 16. 1,
• 2 Chronicles: 35. 1,
• Ezra 6:19, 
• Ezekiel: 45. 21. 

It is interesting to note that at least once the Feast of Unleavened  Bread has been joyfully extended for another 7 days [2 Chronicles: 30. 23]It was also at this Passover and Unleavened  Bread Feast that we find Hezekiah praying over the people who were unclean after they too joined in the Passover.
 So when is the first month?

THAT is an interesting question.
If we were to go by the calendar commonly used in North America, the first month would have been January, yet Passover is clearly marked as falling on April 23rd to 24th in 2016.
This is during the first month of the Assyria Calendar - also known  as Nisan. This is a joy -filled month and agricultural beginning of harvest in Israel.
How are we to eat the Passover Meal? 
Traditionally, Passover has been eaten lolling and reclining on the left side one another in an attitude of sumptuousness - to mimic the Egyptian slave owners.
We find reference to Christ and His disciples eating Passover in this manner:

Matthew: 26. 19 -20 

So the disciples did as Jesus had directed them and prepared the Passover. When evening came, Jesus was reclining at the table with the Twelve. - Bible Offline

However, people were commanded to eat the first Passover in haste  like this:

 Exodus 12:11 "...with your cloak tucked into your belt, your sandals on your feet and your staff in your hand. Eat it in haste; it is the Lord's Passover. - Bible Offline"

How did this change?
I  haven't been able to find anything in Biblical writings about this changing, but several rabinnical scholars have written that it is to demonstrate that we are free and no longer slaves.
If you find any biblical grounds for this change I'd love to hear about it.
Will you be ridding your home of yeast  in preparation for Passover?
What Passover  traditions does your family have?
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Suzanne