a very brief answer ....
Jesus was in Jerusalem during the time of the Jewish Passover (8 days long) - which was dictated by the Jewish Calendar Dates . The last Supper was (by that Calendar) Nisan 14 - bearing in mind, of course, that the Jewish day begins and ends at Sunset - not Midnight. So, the real Last Supper date is dictated by the date in the Gregorian Calendar that corresponds to the Jewish Nisan 14 - so, this year the last supper date would have been 14th April, but, that date not being a Friday, the date was pushed forward to the next Friday / Saturday / Sunday.
from wiki:
Months[edit]
The Hebrew calendar is a lunisolar calendar, meaning that months are based on lunar months, but years are based on solar years.
The calendar year features twelve lunar months of twenty-nine or thirty days, with an intercalary lunar month added periodically to synchronize the twelve lunar cycles with the longer solar year. (These extra months are added seven times every nineteen years. See Leap months, below.) The beginning of each Jewish lunar month is based on the appearance of the new moon.[12] Although originally the new lunar crescent had to be observed and certified by witnesses,[13] the moment of the new moon is now approximated arithmetically. The mean period of the lunar month (precisely, the synodic month) is very close to 29.5 days. Accordingly, the basic Hebrew calendar year is one of twelve lunar months alternating between 29 and 30 days:
No. Hebrew calendar Length
1 Nisan 30
2 Iyar 29
3 Sivan 30
4 Tammuz 29
5 Av 30
6 Elul 29
7 Tishrei 30
8 Marcheshvan 29
9 Kislev 30
10 Tevet 29
11 Shevat 30
12 Adar 29/(30)
Total 354/(355)
In leap years (such as 5771) an additional month, Adar I (30 days) is added after Shevat, while the regular Adar is referred to as "Adar II."
The insertion of the leap month mentioned above is based on the requirement that Passoverโthe festival celebrating the Exodus from Egypt, which took place in the springโalways occur in the [northern hemisphere's] spring season. Since the adoption of a fixed calendar, intercalations in the Hebrew calendar have been assigned to fixed points in a 19-year cycle. Prior to this, the intercalation was determined empirically:
The year may be intercalated on three grounds: 'aviv [i.e.the ripeness of barley], fruits of trees, and the equinox. On two of these grounds it should be intercalated, but not on one of them alone.Importance of lunar months[edit] From very early times, the Mesopotamian lunisolar calendar was in wide use by the countries of the western Asia region. The structure, which was also used by the Israelites, was based on lunar months with the intercalation of an additional month to bring the cycle closer to the solar cycle.
Num 10:10 stresses the importance in Israelite religious observance of the new month (Hebrew: ืจืืฉ ืืืืฉ, Rosh Chodesh, "beginning of the month"): "... in your new moons, ye shall blow with the trumpets over your burnt-offerings..." Similarly in Num 28:11. "The beginning of the month" meant the appearance of a new moon.
According to the Mishnah and Tosefta, in the Maccabean, Herodian, and Mishnaic periods, new months were determined by the sighting of a new crescent, with two eyewitnesses required to testify to the Sanhedrin to having seen the new lunar crescent at sunset.[16] The practice in the time of Gamaliel II (c. 100 CE) was for witnesses to select the appearance of the moon from a collection of drawings that depicted the crescent in a variety of orientations, only a few of which could be valid in any given month.[17] These observations were compared against calculations.[18]
At first the beginning of each Jewish month was signaled to the communities of Israel and beyond by fires lit on mountaintops, but after the Samaritans began to light false fires, messengers were sent.[19] The inability of the messengers to reach communities outside Israel before mid-month High Holy Days (Succot and Passover) led outlying communities to celebrate scriptural festivals for two days rather than one, observing the second feast-day of the Jewish diaspora because of uncertainty of whether the previous month ended after 29 or 30 days.[20]
In his work Mishneh Torah (1178), Maimonides included a chapter "Sanctification of the New Moon", in which he discusses the calendrical rules and their scriptural basis. He notes,"By how much does the solar year exceed the lunar year? By approximately 11 days. Therefore, whenever this excess accumulates to about 30 days, or a little more or less, one month is added and the particular year is made to consist of 13 months, and this is the so-called embolismic (intercalated) year. For the
year could not consist of twelve months plus so-and-so many days, since it is said: throughout the months of the year (Num 28:14), which implies that we should count the year by months and not by days."[21]
