Question: May one begin to eat Seuda Shelishit (the third Shabbat meal) after sunset of Motza’ei Shabbat?
Answer: Let us first introduce this topic by discussing some general ideas. On Motza’ei Shabbat, one is obligated to recite Havdala on a cup of wine, as is printed in all Siddurim. One must be sure to recite Havdala after halachic nightfall which for purposes of Motza’ei Shabbat is approximately half an hour after sunset (in the United States one should wait until approximately forty minutes after sunset).
The Gemara (Pesachim 105a) states that once Shabbat has concluded, one may not eat anything until one recites Havdala on a cup of wine. Our Sages were so stringent about this issue that they said that one who eats before reciting Havdala on a cup of wine shall die of Askara (an agonizing illness which attacks the respiratory system). All Poskim concur that once Shabbat had concluded, one may not eat anything until one performs Havdala.
Nevertheless, the Gemara states that if one began eating Seuda Shelishitbefore Shabbat had ended and the meal continues past the time Shabbat has concluded, one need not stop one’s meal and recite Havdala, for this is not respectful to the Shabbat. Rather, as long as one has begun the meal permissibly, one may continue it. After one has finished one’s meal, one should pray Arvit and then recite Havdala on a cup of wine.
Based on the above, it would seem that one may not begin eating Seuda Shelishit after sunset of Motza’ei Shabbat, for at this point, one may not taste anything until one performs Havdala. Thus, one will have missed out on eating the third Shabbat meal since he has delayed it until after sunset.
Nevertheless, this is not quite so simple, for our Sages tell us in the Gemara that “one may not eat until one performs Havdala,” which would imply that the prohibition to eat only takes effect at the appropriate time for Havdala on Motza’ei Shabbat. One may only perform Havdala on Motza’ei Shabbat when Shabbat has truly concluded, i.e. at halachic nightfall which is thirteen and a half seasonal minutes after sunset, and not immediately at sunset. Indeed, the Turei Zahav (Chapter 299) writes that according to Maran Ha’Shulchan Aruch, the prohibition to eat before Havdala takes effect at nightfall and not at sunset. There are, nevertheless, those that disagree and write that one may not eat anymore immediately at sunset.
Thus, halachically speaking, it is proper to begin eating Seuda Shelishitbefore sunset of Motza’ei Shabbat. Once sunset has passed, one may not eat anything until reciting Havdala. Nevertheless, if, for whatever reason, one did not have a chance to begin eating Seuda Shelishit before sunset, one may begin eating this meal until approximately fifteen minutes after sunset; at that point, since one has begun eating the meal permissibly, one may continue this meal for as long as one wishes.