The "earliest" late Pesach (as seen in the chart) is year 14 of the cycle. In 5790 (2030) Pesach will be 18 April, which is still within the allowable gap before the equinox. That Pesach will not be after April 21 until 15th of Nisan, 6664 = Tue, 22 April 2904. Thus we can theoretically use the current fixed calendar until then.
As of now, Rosh Hshana 5785 will be on 3 October 2020. Thirty days earlier would be on Tuesday, 3 September 2024. Both Cheshvan and Kislev will be 29 day months in 5784. Had we needed to (such as Rosh Hashanah occurring on Wednesday) we could have added the extra day to move it. I will assume for this post that we do not need to move it from Tuesday to Thursday (though it could have been done). Moving the date by thirty days, will change the day of the week by two days.
The following Chanukah, which is scheduled for 26 December, would then occur on Thursday December 26, would then be Tuesday November 26, while Thanksgiving would be on 21 November. With the calendar shift, the first day of Chanukah would be on November 25.
The next Pesach occurs on April 13, 2025 according to the current fixed calendar. The shift would move it to March 13, which is before the equinox. As a result, the Sanhedrin would probably declare 5785 as a leap year moving Pesach back to April 15. The following year would then be back according to the fixed schedule, except that the cycle would have moved. Similar results can be expected with the new calendar, except that the Sanhedrin would keep track.
Now lets examine the entire cycle for that year (cycle 304). The first year of that cycle is 5776 which is 2015/2016.
Note that moving Rosh Hashannah by thirty days will cause the day of the week to change by two days. If Rosh Hashanah is on Tuesday, moving it back would cause it to occur on Sunday, which is not allowed. This would mean that it needs to be moved ahead to Monday or back to Shabbos by modifying the "extra" day that could be added in Chesvan or Kislev. Similarly, moving it ahead by thirty days would cause the problem to occur Rosh Hashanah is on Monday and would move ahead to Wednesday. For the following table, I will assume that the equinox on 21 March is the critical date. Thus, if Pesach occurs after 21 April, there would need to be a modification. Those years would shown as yes in the modification column. The following year would be left blank as it would probably need to become a leap year to account for the fact that Pesach would be "too early".
| Year | Secular | Cycle | leap year | Pesach | Modify | Next Rosh Hashanah |
|---|---|---|---|---|---|---|
| 5776 | 2016 | 0 | yes | Sat, 23 April 2016 | yes | Mon, 3 October 2016 |
| 5777 | 2017 | 1 | no | Tue, 11 April 2017 | Thu, 21 September 2017 | |
| 5778 | 2018 | 2 | no | Sat, 31 March 2018 | no | Mon, 10 September 2018 |
| 5779 | 2019 | 3 | yes | Sat, 20 April 2019 | no | Mon, 30 September 2019 |
| 5780 | 2020 | 4 | no | Thu, 9 April 2020 | no | Sat, 19 September 2020 |
| 5781 | 2021 | 5 | no | Sun, 28 March 2021 | no | 7 September 2021 |
| 5782 | 2022 | 6 | yes | Sat, 16 April 2022 | no | 26 September 2022 |
| 5783 | 2023 | 7 | no | Thu, 6 April 2023 | no | Sat, 16 September 2023 |
| 5784 | 2024 | 8 | yes | Tue, 23 April 2024 | yes | Thu, 3 October 2024 |
| 5785 | 2025 | 9 | no | Sun, 13 April 2025 | Tue, 23 September 2025 | |
| 5786 | 2026 | 10 | no | Thu, 2 April 2026 | no | Sat, 12 September 2026 |
| 5787 | 2027 | 11 | yes | Thu, 22 April 2027 | yes | Sat, 2 October 2027 |
| 5788 | 2028 | 12 | no | Tue, 11 April 2028 | Thu, 21 September 2028 | |
| 5789 | 2029 | 13 | no | Sat, 31 March 2029 | no | Mon, 10 September 2029 |
| 5790 | 2030 | 14 | yes | Thu, 18 April 2030 | no | Sat, 28 September 2030 |
| 5791 | 2031 | 15 | no | Tue, 8 April 2031 | no | Thu, 18 September 2031 |
| 5792 | 2032 | 16 | no | Sat, 27 March 2032 | no | Mon, 6 September 2032 |
| 5793 | 2033 | 17 | yes | Thu, 14 April 2033 | no | Sat, 24 September 2033 |
| 5794 | 2034 | 18 | no | Tue, 4 April 2034 | no | Thu, 14 September 2034 |
| 5795 | 2035 | 0 | yes | Tue, 24 April 2035 | no | Thu, 4 October 2035 |
The following tableshows what would happen if the Sanhedrin "skipped" a leap year that has Purim fall after the vernal equinox so that Pesach is after April 21. The following year would then be made a leap year so that Pesach would not fall "too early". In order to estimate the date of Pesach, it will be assumed that adding the thirty days just compensates for the thirty day subtraction, so that Pesach will occur on the same day as that shown on the fixed calenday, allowing for any Cheshvan Kislev modification. This will also account for Rosh Hashanah. Instead of calculating the following Rosh Hashannah according to the full set of rules, I will subtract the 30 days and adjust if necessary by modifying the preceding Cheshvan Kislev calculation so that it does not occur on Sunday, Wednesday, or Friday. I will otherwise assume that Cheshvan and Kislev appear as they would in the current fixed calendar. The modification value will be -1 either Chesvan or Kislev must be changed from 30 to 29 days, +1 if either is changed from 29 to 30 days, and 0 if it is left as set in the current fixed calendar. It will be left blank if Rosh Hashanah did not change from that in the current fixed calendar.
| Year | Secular | Cycle | leap year | Pesach | Modify | Next Rosh Hashanah |
|---|---|---|---|---|---|---|
| 5776 | 2016 | 0 | no | Wed, 23 March 2016 | 0 | Sat, 3 September 2016 |
| 5777 | 2017 | 1 | yes | Tue, 11 April 2017 | Thu, 21 September 2017 | |
| 5778 | 2018 | 2 | no | Sat, 31 March 2018 | Mon, 10 September 2018 | |
| 5779 | 2019 | 3 | yes | Sat, 20 April 2019 | Mon, 30 September 2019 | |
| 5780 | 2020 | 4 | no | Thu, 9 April 2020 | Sat, 19 September 2020 | |
| 5781 | 2021 | 5 | no | Sun, 28 March 2021 | 7 September 2021 | |
| 5782 | 2022 | 6 | yes | Sat, 16 April 2022 | 26 September 2022 | |
| 5783 | 2023 | 7 | no | Thu, 6 April 2023 | Sat, 16 September 2023 | |
| 5784 | 2024 | 8 | no | Sat, 23 March 2024 | 0 | Tue, 3 September 2024 |
| 5785 | 2025 | 9 | yes | Sun, 13 April 2025 | Tue, 23 September 2025 | |
| 5786 | 2026 | 10 | no | Thu, 2 April 2026 | Sat, 12 September 2026 | |
| 5787 | 2027 | 11 | no | Mon, 22 March 2027 | 0 | Thu, 2 September 2027 |
| 5788 | 2028 | 12 | yes | Tue, 11 April 2028 | Thu, 21 September 2028 | |
| 5789 | 2029 | 13 | no | Sat, 31 March 2029 | Mon, 10 September 2029 | |
| 5790 | 2030 | 14 | yes | Thu, 18 April 2030 | Sat, 28 September 2030 | |
| 5791 | 2031 | 15 | no | Tue, 8 April 2031 | Thu, 18 September 2031 | |
| 5792 | 2032 | 16 | no | Sat, 27 March 2032 | Mon, 6 September 2032 | |
| 5793 | 2033 | 17 | yes | Thu, 14 April 2033 | Sat, 24 September 2033 | |
| 5794 | 2034 | 18 | no | Tue, 4 April 2034 | Thu, 14 September 2034 |
This becomes the new cycle. As can be seen several of the leap years would not require modification. It is possible that the Sanhedrin would not want to change the cycle until the date of Pesach moves farther from the equinox so that all the leap years of the cycle occur after the equinox. As I said earlier, we could use the current calendar until 6664 (2904). The "latest" leap year Pesach on 23 April is only 5 days away from the "earliest" leap year Pesach so that the determination based on "season" and the physical aspects of the year would be more significant than the astronomical calculation.
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