Why do we sometimes repeat shmoneh esrei when we forget yaaleh v'yavo and sometimes not?

Question

It is my understanding that we do not repeat shmoneh esrei when we forget yaaleh v'yavo by maariv on rosh chodesh because maariv is a reshus. Based on this premise why do we repeat shmoneh esrei when we forget yaaleh v'yavo by maariv on chol hamoed?

Answer 1

The Talmud in Berachot 30b explains why we do not repeat Shemoneh Esrei on Rosh Chodesh evening:

אמר רב ענן אמר רב טעה ולא הזכיר של ר"ח ערבית אין מחזירין אותו לפי שאין בית דין מקדשין את החדש אלא ביום אמר אמימר מסתברא מילתא דרב בחדש מלא אבל בחדש חסר מחזירין אותו א"ל רב אשי לאמימר מכדי רב טעמא קאמר מה לי חסר ומה לי מלא אלא לא שנא

R. 'Anan said in the name of Rab: If one forgot and made no mention of New Moon in the evening prayer, he is not made to repeat, because the Beth din sanctify the New Moon only by day. Amemar said: This rule of Rab seems right in a full month, but in a defective month he is made to repeat. Said R. Ashi to Amemar: Let us see: Rab gave a reason, so what does it matter whether it is full or defective? In fact there is no difference.

(Soncino translation)

This reason would not be applicable to Chol Hamoed, and indeed in Shabbat 24a the Talmud cites a Beraita that states that one must repeat Shemoneh Esrei on Chol Hamoed for all three prayers:

דתני ר' אושעיא ימים שיש בהן קרבן מוסף כגון ר"ח וחוש"מ ערבית ושחרית ומנחה מתפלל שמונה עשרה ואומר מעין המאורע בעבודה ואם לא אמר מחזירין אותו

For R. Oshaia taught: On those days when there is an additional offering, viz., New Moon and the weekdays of Festivals10 at the Evening, Morning and Afternoon [services] the Eighteen [Benedictions] are recited, and the nature of the occasion is inserted in the 'Abodah; and if one does not insert it, he is turned back;

(Soncino translation)

These Talmudic quotes are codified in Shulchan Aruch in O.C. 422:1 and O.C. 490:2 respectively.