Who should observe Isru Chag?

Question

I'm trying to understand Isru Chag. While there is indeed a source in Psalms for the words, and while the Gemara indeed derives a notion of extra celebration, there seems to be little about the scope of the day.

What troubles me is that the codification of Isru Chag via the Ramo takes place in galut -- post-15th century. The last day of Passover would already have been logically obviated and maintained only as a derobonnon practice. Therefore, the deriving of a day as an extension of the holiday would already have been satisfied by the addition (and maintaining) of an entire second day of Yom Tov. Isru Chag would still have value in Israel because otherwise, nothing would be "bound" to the end of the holiday. But, outside of Israel, we are already "binding" a second day of Yom Tov.

We know that the last day of Passover is not really part of the Biblically-mandated holiday. So the derivation of Isru Chag via a line in Psalms (written about the celebration before yom tov sheini shel galuyos existed) to create another day seems unnecessary.

So, shouldn't Isru Chag be a practice only for Ashkenazim in Israel?

Answer 1

The source of Isru Chag is from Chagiga 18b. The Gemara says that, when Shavuoth falls on Shabbat, the Korbanot of Chagiga and Re'iya were brought (preferably) on Sunday. (They can be bought the entire week, but the earliest - and best - time was on Sunday.)

This Sunday is called יוֹם טְבוֹחַ - the day of slaughtering.

As a result, the day after Shavu'oth must be celebrated as an Isru Chag, with all of its laws: e.g. no fasting, eulogizing, or Tachanun.

The Aruch Hashulchan in סימן תכט - דיני חודש ניסן explains that even for the Pessach and Sukkoth there is a reason to celebrate Isru Chag, since the Korban Chagiga was eaten for 2 days (and the night between). So those Korbanot brought on the last day of Yom Tov were still being eaten on Isru Chag.

ה: ונוהגים להרבות קצת באכילה ושתייה ביום שאחר החג, והוא ''אסרו חג''. והמנהג שלא להתענות בו, דנחשב קצת כיום טוב. דאסרו חג שבועות – וודאי היה יום טוב, שבו היה יום טבוח, ותפסו ליום טוב כל אסרו חג גם של פסח ושל סוכות. ובסוכה (מה ב): אמרו כל העושה איסור לחג באכילה ושתייה – מעלה עליו הכתוב כאלו בנה מזבח, והקריב עליו קרבן, שנאמר: ''אסרו חג בעבותים עד קרנות המזבח''. ופירש רש''י בלשון אחד דא''אסרו חג'' קאי, עיין שם. ‏

ונראה לי דהכי פירושו: דהנה שמחת החג, כשמכוין לשם שמים – עיקר השמחה בהקרבנות, שלמי שמחה. ואם בהתפעלות נפשו הקשורה לה' שמח גם כן ביום שלאחריו, ואוכל מהשלמים של אתמול, דשלמים נאכלים לשני ימים ולילה אחד, הוי התוספות דכאילו לבד הקרבנות – בנה גם כן את המזבח.‏

Reb Uri Weinberg זצ"ל said that, in Israel, there's an additional reason not to say Tachanun (or למנצח) on Isru Chag. This is because, in the Diaspora, it is still Yom Tov.