Question:

How will the Jewish calendar be set in the times of Moshiach, if the moon will be like the sun?

Answer:

To answer this question, let us first review some concepts related to the Jewish calendar. The Jewish calendar is established by the lunar cycle. The moon, however, has no light of its own, and only reflects the light of the sun. The monthly waxing and waning of the moon is a result of its cycle of approaching and receding from the sun.

Regarding the future times, the verse states: "The light of the moon will be like the light of the sun (Isaiah 30:26). The moon will become equal to the sun and will be a source of its own light, as it was when it was first created. As a result, there will no longer be the concept of a waning moon; the moon will always be full, just like the sun. If then, however, how will the concept of "month" be established?

In other words, there is a mitzvah in the Torah to sanctify the new moon upon the testimony of two witnesses who claim to have seen the new moon. In the future, there will never be a "new moon," since the moon will never appear to be smaller. How, then, will we fulfill the mitzvah of sanctifying the new moon?

Similarly, the concept of "leap year," which was instituted to maintain a correlation between the solar and lunar year, will become obsolete.

However, the Lubavitcher Rebbe explains that it will be thus: In the book of Tanya, Rabbi Schneur Zalmen of Liadi explains that according to our sages, there will be two stages in Redemption: Before the resurrection of the dead, and afterwards. After the resurrection, the normal structure of mitzvah observance will be nullified, as the Talmud states: "Mitzvot will be nullified in the future."

In keeping with this, it is possible to say that the prophecy of the moon becoming like the sun will not be realized until the second stage. At that point, mitzvah observance as we know it will no longer exist, and there will be no need for sanctifying the moon or making a leap year.

Sources: Niddah 61a. Sefer Hamitzvos, root 3. Tanya, Igeres Hakodesh 26. Toras Menachem 5743, vol. 3, p. 1528.