How can the mourner remarry within one month, if he has to wait two regalim?

Question

Moed katan (21b):

מתה אשתו ונשא אשה אחרת — אינו רשאי ליכנס לביתו לדבר עמו ]תנחומין, [כדי שלא לפגוע באשה החדשה. If it was the mourner’s wife who died and he married another woman within thirty days of his first wife’s death, one may not enter his house to speak words of consolation with him, [so as not to offend his new wife].

The Gemara teaches that one may not console someone in his first thirty days of mourning whose wife died and he remarried, so as not to offend his new wife. And yet:

Moed katan (23a):

[תנו רבנן [שנו חכמים]: כל שלשים יום [שלאחר כל אבל אסורים לנישואין. [ואם] מתה אשתו — אסור לישא אשה אחרת עד שיעברו עליו שלשה רגלים [מאז מותה.] ר' יהודה אומר: רגל ראשון ושני — אסור, שלישי — מותר. The Sages taught another baraita: During the entire thirty-day [period of mourning, it is prohibited] to marry. If one’s wife died, it is prohibited to marry another wife until three Festivals pass [since her death]. Rabbi Yehuda says: Until the first and second Festivals have passed, he is prohibited from marrying; before the third Festival, however, he is permitted to do so. [Steinzaltz in brackets, sefaria’s translation]

We learn in the B’raisa that one may not remarry for at least two regalim, which is at least a 50 days (if we go according to rabbi Yehuda, and his wife dies the day before Pesach), or over a month! So how does the first case even make sense, that he remarried within 30 days?

Answer 1

Rashi on 21b points to the continuation of the gemara on Moed Katan 23a which provides the answer

ואם אין לו בנים מותר לישא לאלתר משום ביטול פריה ורביה הניחה לו בנים קטנים מותר לישא לאלתר מפני פרנסתן

And if he does not have children, he is permitted to marry another wife immediately due to the need to not neglect the mitzva to be fruitful and multiply. (R Steinsaltz: Since he has not yet fulfilled the mitzva of procreation, he is still required to marry a wife. Any delay might result in a lost opportunity for marriage.)

Similarly, if his wife died and left him young children, he is permitted to marry another wife immediately, so that she might take care of them.

What does marry immediately mean? SA YD 392:2 writes it is after the shiva.

Answer 2

Art Scroll Gemara Moed Katan 21b2 note 14 says

As we will see later, there are circumstances when a man may remarry within sheloshim, as, for example, when he has no children (and thus has not yet fulfilled the mitzvah of procreation) Rashi, (from the gemara below 23a)

Also consider the kohen gadol before Yom Kippur who must remarry in order to perform the service.

The gemara on Daf 23a (Art Scroll 23a2) explicitly says

ואם אין לו בנים מותר לישא לאלתר

And if he does not have children He is permitted to marry immediately

משום ביטול פריה ורביה

on account of the cessation from the obligation of being fruitful and multiplying that would result from waiting to remarry ¹⁸

¹⁸ Some maintain that immediately in this context means right after shivah, whereas others explain that it means right after sheloshim (see Rambam, Rosh par. 48 and Ritva) Shulchan Aruch (Yoreh Deah 392:2) rules that the mourner who does not have children may perform eirusin immediately, adn may perform nisuin and consummate the marriage after shivah