המועד הנשכח: אסרו חג

The day after יו"ט we are נוהג to increase our eating and drinking somewhat, that day being אסרו חג [1].

This concept is based on ר ירמיה (quoting ר שמעון בו יוחאי) and ר יוחנן (quoting ר שמעון המחוזי in turn quoting ר יוחנן המכותי) who taught that whoever makes an 'איסור' to the חג through eating and drinking then the פסוק considers them as though then have built a מזבח and brought קורבנות thereon. This is derived from the פסוק that says "אסרו חג בעבותים עד קרנות המזבח", meaning that we should tie up the festival with ropes until the horns of the מזבח.

רשי explains the דרשה to mean that one who enjoys the day after יו"ט, thereby 'binding' the יו"ט to themselves, is as though they have brought עבותים, literally ropes but here interpreted allegorically as fat animals, on the מזבח [2].

ערוה"ש explains further [3] that the joy of a festival is the קורבן שלמים that is then brought. If the excitement of bonding with 'ה spills over into the day following יו"ט, and a person eats then from the שלמים of the previous day (because שלמים are eaten for two days and one night) then that overflow is considered as though one has built the מזבח.

[1] רמא או"ח תכט:ב. See the discussion in the שו"ע there with regards to fasting on אסרו חג, a purely halachic matter that is beyond the scope of this forum.

[2] סוכה מה. ועיין רשי שם ד"ה איסור לחג, באכילה ושתיה, בעבותים

The above is according to רשי's interpretation. The מהרשא there, however, has the exact opposite explanation, namely that one who stops themselves from indulging in excess food and drink is as though they have offered that extra food and drink that was not partaken of on the מזבח. The consensus of the שו"ע appears to follow רשי's interpretation.

[3] או"ח שם סעיף ה