Resources for Chagigah daf 18
1. There is a big מחלוקת ראשונים as to whether חול המועד is אסור במלאכה מדרבנן or מדאורייתא. There are three basic approaches: one is רש"י here and the ריטב"א in מועד קטן דף ב in ד"ה ואיכא who say that the simple reading of our גמרא is that it is a דין דאורייתא since the גמרא is darshening a פּסוק in the תורה and says the פּסוק can’t be referring to חול המועד since one can’t do מלאכה on חול המועד. However, תוספות on our דף in ד"ה חולו של מועד says that the איסור מלאכה on חול מועד is only a דין דרבנן. As to the conflicting משמעות from our גמרא, he says that the גמרא just means that if the פּסוק would be saying that you are allowed to harvest on חול המועד, it’s not מסתבר that the חכמים would have forbade it. See the מנחת חינוך in מצוה שכ"ג who says that תוספות means like the ט"ז in יו"ד סימן קי"ז who says that anytime the תורה explicitly says something is permitted, the חכמים do not have the authority to come and forbid it. This is why מילה is still מותר on שבת even though we should have made a גזרה to not do it since the מוהל may come to carry the knife on שבת (like we don’t allow blowing shofar on ר"ה when it falls out on שבת because someone might come to carry the shofar). The reason the חכמים could not forbid מילה on שבת is because the תורה explicitly allows it. Similarly, if one had to read the פּסוק as saying one could do מלאכה on חול המועד, then חז"ל would not have forbade it.
The third opinion in the ראשונים is the רמב"ן brought by the נימוקי יוסף at the beginning of מ"ק there he says a “פּשרה”: the מלאכות that aren’t לצורך המועד and aren’t דבר האבד are דאורייתא, and the rest are דרבנן. See the משכנות יעקב in או"ח סימן ל"ז who wants to prove that תוספות must be correct that מלאכת חול מועד is only דרבנן for the following reason: we pasken that cooking on יו"ט for שבת is מותר מדאורייתא because of the principle of “הואיל” which means that since guests may come, you can say you are cooking on יו"ט in case they come. If so, all מלאכה done on חול המועד would be מותר מדאורייתא since maybe something will happen that will require you to do that מלאכה on יו"ט. However, see the Netziv in העמק שאלה in דברים שאילתא ק"ע who disagrees and say that תוספות himself on the next דף says that we only הואיל if the possibility could come true. Where it is impossible (such as where you start cooking two seconds before יו"ט is over) then we don’t say הואיל.
2. The גמרא says that ר' טרפון did not allow הספּידים on the day after שבועות because it was the יום טבוח. See the תוספות רי"ד here who says that this story took place after the חורבן בית המקדש! If so, it means that even nowadays, יום טבוח is אסור בהספּידים. He says this even applies in חוץ לארץ where we have two days of יו"ט since the second day is really just a ספק first day. This is indeed the one of the main reasons given for איסרו חג. However, according to this, only שבועות would really have an איסרו חג since the other holidays have חול המועד to bring the קרבנות of the רגל. However, there is another reason brought in the בית יוסף in סימן תצ"ד which is based on the גמרא in סוכה דף מ"ה ע"ב which says “כל העושה איסור לחג באכילה ובשתיה מעלה עליו הכתוב כאילו בנה מזבח”. That would apply to all holidays.
Rabbi Millman's Marei Mekomos Halacha
Rabbi Azriel Katz - Meforshim Overview
Rabbi Yechiel Grunhaus - Points to Ponder
New Daf Hashavua newsletter - Shavua Matters