What is the reason Sukkos is for 7 days? Pesach I understand because the Jews left on 15 Nissan and the Exodus culminated seven days later with Krias yam suf 7 days later. Kabbalas HaTorah was one day so it makes sense that Torah has it celebrated for one day. Was there something that happened for seven days around this time, and if not, how can it be understood why it should be 7 days?
You are asking an important and interesting question. There must be many answers to this. So far I found these two answers. See Midrash Tehillim 17-5 That the seven days are connected to the posuk שובע שמחות את פניך, “we greet you face full of happiness”. Says the Midrash we read the word, שבע, not only as “full”, but also as “seven” שבע. That we have seven days of festivities and being happy with Hashem because on Succos we have seven mitzvos, Succah, the four minim, nisuch hamayim, and Simchas Beis Hashoeiva.
Another answer is found in the Abarbenel at the end of Parshas Reah (see link אברבנאל על סוכות ) Says that the reason Succos is seven days is to celebrate with the fact that the years crops are already gathered, and now the farmer is not worried about his livelihood, therefore we can rejoice together with Hashem without worry. He also says that the three Festivals are for three presents that hashem has given us. Going out of Mitzraim ( Pesach) Getting the Torah ( Shavous) and going into Eretz Yisroel ( Succos). He says that the seven days are for the seven species of fruit that Eretz Yisroel is known for.
Have a gut moed