Of all the fast days on the Jewish calendar, Yom Kippur is the strictest. It is the only fast mandated by the Torah itself, not the books of Prophets. Also, the other fasts were established to commemorate the destruction of Jerusalem and the Holy Temple, and will be nullified with the final Redemption. However, Yom Kippur will remain in force, like all the other mitzvot in the Torah.

However, the possibility still exists that eating on Yom Kippur will be permissible, and not just for health reasons. If Moshiach should come in the days immediately preceding Yom Kippur and the Holy Temple rebuilt, the fast will be suspended to celebrate its inauguration.

The first Holy Temple was inaugurated by King Solomon on the 8^th of Tishrei, and was celebrated for the next 14 days. In that year they did not fast on Yom Kippur, which fell right in middle of the festivities. The sages worried that they had erred in the matter, but a heavenly voice proclaimed that all those who had participated in the inauguration of the Temple would merit the World to Come.

The celebration of the rebuilding of the third Holy Temple will be even greater than the first two, as the verse states, “The honor of the final Temple will be greater than the first” (Chaggai 2:9). Therefore, if Yom Kippur will fall during the week of inauguration there will be no need to fast, in honor of the great joy of the occasion.

 (Hitvaaduyot 5749, vol. 1, p. 46; talk of the Lubavitcher Rebbe, King Moshiach, Erev Yom Kippur 5752)