The Above Calendar is based on the wonderful insight, which is SIMPLY SCRIPTURE in CONTEXT, by the Karaites, http://www.karaite-korner.org/passover.shtml
Passover and Unleavened Bread
These days one hears much about the "Holiday" of Passover and even we Karaites refer to it often. But in the Hebrew Bible there is no such holiday! In the Tanach "Passover" is the name of a sacrifice, while the holiday is called Hag HaMatzot ("Feast of Unleavened Bread"). Thus in the verse: "Draw out and take a lamb according to your families, and slaughter (KJV: kill) the passover." (Ex 12,21). In this verse the "Passover" is the lamb that is to be sacrificed by slaughtering and eating it. Similarly in Ex 12,26-27: "...when your children shall say to you, What mean ye by this service? And you shall say, It is the sacrifice of YHWH's passover". The service of slaughtering the lamb and eating it is called "the sacrifice of YHWH's passover". This is also the meaning of Passover in the verse: "In the fourteenth day of the first month between the two evenings is YHWH's passover" (Lev 23,5). And again in Dt 16,1: "Observe the month of the Abib, and perform the passover unto YHWH your God". To "perform" or "keep" the Passover (in Hebrew literally "do the Passover") means to bring the Passover sacrifice and eat it. It is only in post-Biblical times that the word Passover took on the new meaning of referring to the Holiday on which the sacrifice was eaten and not to the sacrifice itself. Today we often hear of the "Passover Holiday" and "Hag Ha-Pessach" both of which are post-Biblical inventions. In the Tanach the Holiday is called Hag HaMatzot which means: "Feast of Unleavened Bread".
When was the Passover Sacrifice Brought?
The Torah commands us: "In the First Month on the fourteenth day of the month, between the two evenings [Hebrew: "Bein Ha'arabayim"], is the Passover [Sacrifice] to YHWH." In biblical Hebrew, the word "evening" (Ayin-Resh-Bet) indicates both the "early part of the night" as well as the actual "onset of evening". In the expression "between the two evenings" the first "onset of evening" is sunset (when the disk of the sun disappears) while the second "onset of evening" is the disappearance of the last rays of the sun and the onset of total darkness. The expression "between the two evenings" is used interchangeably with the term "Ba-Erev" (literally: "at evening") which itself refers to the "onset of the evening". For example, in the incident of the Manna it is written (Ex 16,11-13):
"I have heard the complaints of the Children of Israel; speak to them saying 'Between the two evenings you shall eat meat'... And it was at evening that the quail rose up and covered the camp.'"
We see in this passage that an event predicted as happening "between the two evenings" is said to have happened "at evening". The meaning of "at evening" itself can be learned from the verse "... you shall slaughter the Passover [sacrifice] at evening, at sunset" (Dt 16,6). We see in this verse that "at evening" and "at sunset" are interchangeable expressions (used in "apposition").
To summarize, the Torah describes the time of the Passover Sacrifice with three different expressions: "At Sunset", "At Evening", "Between the Two Evenings". All three of these terms refer to the early evening, shortly after sunset.
Beginning or End of the 14th?
The Torah commands that the Passover sacrifice be brought "In the First Month on the Fourteenth Day of the Month between the two evenings" (Lev 23,5). It is unclear from this verse whether what is being referred to is the period of dusk at the beginning of the 14th or the period of dusk at the end of the 14th. Lev 23:6 continues that the Feast of Unleavened Bread is "on the Fifteenth Day of this month". From this verse it appears that the Passover Sacrifice is to be brought at sunset at the end of the 14th and eaten on the night of the 15th. This is confirmed by Dt 16:4, which commands us regarding the Passover Sacrifice: "and there shall not remain of the meat that you slaughter at evening on the first day until the morning." We see that the entire Paschal lamb must be consumed on the following night it is slaughtered and none of it may be left over until the morning (see also Ex 12:10,22). For our purposes what is significant is that the verse describes the Passover sacrifice as being slaughtered "at evening on the first day".
The passage in Dt 16:1-8 is talking about the Feast of Unleavened Bread and there can be no doubt that "the first day" in v.4 refers to the first day of Unleavened Bread. We have already seen in Lev 23:6 that the First Day of Unleavened Bread falls out on the 15th of the First Month. When we look at Lev 23:5-6 and Dt 16:4 together it becomes clear that the Passover Sacrifice is brought at the end of the 14th of the First Month between the two evenings and eaten that same evening on the 15th of the First Month. The period of "between the two evenings" is reckoned as both the end of the 14th (Lev 23:5) and the beginning of the 15th (Dt 16:4)!
It is not unusual for the Torah to refer to "such and such a date at evening" and to mean the evening that ends that day. In Lev 23:27 we learn that the Day of Atonement occurs on the 10th day of the Seventh Month. A few verses later the Torah makes clear what is meant by the 10th day: "and you shall afflict your souls on the ninth of the month at evening, from evening to evening you shall observe your Sabbath" (Lev 23:32). So we see that to fast on the 10th day means to fast from sunset on the 9th until the following sunset. In this verse "the ninth at evening" refers to the onset of evening at the end of the 9th, not the beginning! So the fast of the Day of Atonement on the 10th of the month runs from sunset ending the 9th until sunset ending the 10th (see also Ex 12:18). Similarly, the 14th between the two evenings in v.5 of the same chapter refers to the end of the 14th, not the beginning, as confirmed by Dt 16:4.
"and there shall not remain of the meat that you slaughter at evening on the first day until morning" Dt 16:4