Shemowth 12:18 tells us that from the evening of the 14th day until the evening of the 21st day we should eat matsah. So the question is:
Is it from the start of the 14th day until the start of the 21st day, or from the end of the 14th day until the end of the 21st?
I would argue that the instruction pertains to the beginning of the day. Ereb’s first use is in Ba’reshiyth 1:5 where we read, “And Elohim called the light ‘day’ and the darkness He called ‘night.’ And there came to be evening and there came to be morning, the first day.” So ereb/evening proceeded boqer/mourning in the day. We see this same wording placing ereb before boqer each of the days of creation. So from this perspective ereb is the start of the day.
Ereb like evening in English is an imprecise word. What is the difference between afternoon and evening? There is no set time that afternoon ends and evening begins. Afternoon has a definite start time, but no definite end time. At 5:00pm I could say good evening or good afternoon and both could be applicable.
Wikipedia wrote: Evening in its primary meaning is the period of the day between afternoon and night. Though the term is subjective, evening is typically understood to begin when the temperature has noticeably fallen and other accompanying weather changes have occurred, such as increased wind speed and change in cloud types and sky colour, and lasts until an hour or so after sunset, when maximum darkness has been reached.
Ereb occupies that same subjective time period. And in a world where time clocks weren’t punched and hourly wages weren’t a thing precision was not needed. You closed shop when you had no one coming to buy what you were selling, and you went home when all your work was done. If you worked a field your day ended when it got too dark to work, or you finished what needed to be done. Our world works very differently today, which is why we use a precise hour of the day as the switch between one day and another. I imagine when Yah gave his towrah that most were not worried about a specific end to a day, and that even the something like the Sabbath was not started at a specific time of day, but rather whenever they finished their work and went home for the day that was when their Sabbath began.
It’s easy to see how as time became more and more structured over the centuries how questions of precision in regards to days came up, and that feed the Rabbi’s ability to convince people that they were needed to be the arbiters of such things leading to Rabbinical teaching that evening begins when X number of stars are visible in the sky. It’s the imprecision of Yah’s teaching that they grab onto as their justification, when the fact is if Yahowah intended precision he was fully capable of giving it.
In Shemowth Yahowah could have simply said from the start/end of the 14th to the start/end of the 21st, but he choose to use ereb instead. So I completely understand and have no problem if someone wants to celebrate matsah from the end of the 14th to the end of the 21st.
My reason for choosing to observe it from the start of the 14th until the start of the 21st are:
1) Based on my understanding of the day structure in scripture, using Ba’reshiyth 1 as my guide, it seems that ereb starts a day.
2) If the instruction was for the eating of Matsah to start on the 15th, why even mention the 14th? Why not say from the 15th to the 22nd?
3) Having the eating of Matsah start at the beginning of the 14th serves to even more intertwine Pesach with Matsah.
4) If you removed the yeast from your home at the start of the 14th day then as the Pesach lamb is being slaughtered and prepared so is the matsah that would be eaten at the feast which would begin on the 14th and continue on through the 15th until the morning of the 15th.
Again this is just my reason and my understanding. I look forward to hearing from others.