If Moshiach comes on the night of Erev Yom Kippur, will we fast this Yom Kippur?

 

Just like in the time of Shlomo Hamelech they did not fast on Yom Kippur while they were inaugurating the Beis Hamikdash, we will not fast this year Yom Kippur if he comes tonight, on Erev Yom Kippur. Instead, we will have a big meal of the Livyason and Shor Habor and eat in the Kodesh Hakodoshim, together with a joyous Farbrengen.

 

Source: Sichos Kodesh 5752 Page 63 and on.