I am aware that in order to locate Pentecost correctly, I must first nail down Passover exactly. How can this be done when there are so many views of when it is? Didn’t the Crusades have something to do with calendation?

Answer: Much controversy erupted over the time reckoning for “Easter,” the Passover replacement.  Beginning at Pope Victor I A.D. 193-202, this conflict ensued because of the calendars that the differing factions held to.  Those of the west who followed the Roman Julian calendar were amiable to keeping an “Easter Sunday”, while those of the east followed the Scriptural luni-solar calendar, held tightly to observing “Passover” on the night of the 14th.  This last group became known as Quartodecimans.

Quartodecimanism ("fourteenism", derived from Latin) refers to the practice of fixing the celebration of Passover for Christians on the fourteenth day of Nisan in the Old Testament's Hebrew Calendar (for example Lev 23:5, in Latin "quarta decima"). This was the original method of fixing the date of the Passover, which is to be a "perpetual ordinance. . .

Since the Bible's calendar is luni-solar and the Roman/Western calendar is only strictly solar, it is difficult to calculate Nisan 14 in the western calendar without knowledge of how a luni-solar calendar system works. For various reasons, the Church eventually chose to use a different method from the one that the Jews had used for their Passover.

A controversy arose concerning whether it should also be a resurrection holiday, and thus whether it should instead be celebrated on one particular Sunday each year, which is now the floating holiday that is commonly called Easter Sunday.

Quartodecimanism was popular among Christians in Asia Minor and it is generally believed that this was the method specifically preferred by the followers of John the Apostle, since it was advocated by Polycarp who was a disciple of either John the Apostle or John the Presbyter, assuming they are not the same person.  Theological Dictionary

If we faithfully follow the principle of “solo Scriptura” we can not go wrong.

The Passover Feast is the first annual appointed festival of the year and is always in the spring.  It is always on the 14th day of the first month of Abib as counted from the New Moon day, which according to Scripture is also New Year’s Day.  Passover is always the 6th day of the week and also located on the preparation day.  It is always on or very near the full moon.  The day following Passover is always the seventh-day Sabbath, which is also always a High Sabbath, because it is the day Feast of Unleavened Bread commences.  A side note is that the Hebrew O.T. and Greek N.T. never refer to Passover as “Friday” or to the Resurrection day as “Sunday”. Passover is always referred to as the 6th day of the week and Resurrection day was the fist day of the week.  In the sequence that follows, Passover is the first of the Appointed Feasts which Yahushua the Messiah fulfilled precisely.  (For more details and color illustrations, please refer to the eCourse, Twelve Criteria of the True Crucifixion Date.”)

These are the feasts of Yahuwah, holy convocations which you shall proclaim at their appointed (yearly) times.  On the fourteenth day of the first month at twilight is Yahuwah’s Passover.  Leviticus 23:4-6

The point was raised also regarding the Crusades being fought over calendation.  There is some evidence of this, but not solid proof, that the Islamic nations once followed the luni-solar calendar.  The Oxford English Dictionary gives evidence that the word “Ramadan”, literally means dry and hot, and was at one time connected to a warm harvest season.  However, I believe that just as the calendation of Scripture was messed with, so was theirs, which is likely to have originally been luni-solar.  Due to much pressure the west has followed after Rome and the revolutions of the “sun” only, as in the solar calendar.  In contrast, the Islamic nations began to follow after the “moon” only, in keeping a strictly lunar calendar.  This has caused their “Ramadan” to rotate throughout the year, so that it is often celebrated in winter when no harvest is possible.  Here is why this happens: