The first night of Sukkos we make kiddush after tzeit (Rama 639:3). My understanding is that this is purely because of the bracha of לישב בסוכה which you can't say until it's definitely night (Magen Avraham and Biur Halacha). The Dirshu Mishna Berura quotes someone, I believe it's R' Chaim Brisker, who makes the point that we worry about the possibility (R' Tam?) that accepting Yom Tov does not create a full reality of Yom Tov that would allow you to say this bracha. Otherwise, you could make kiddush and eat, and just make sure to eat a kezayit of bread after tzeit.
Are there situations when you can make kiddush before tzeit?
- You are an woman and not eating in the sukkah, or a Sefardi woman and eating in the sukkah with no לישב בסוכה Does this allow for Kiddush to be made before nightfall?
- You're a man and will not be able to eat in the sukkah, e.g.a hurricane is building up and its pikuach nefesh if you go in after nightfull, should one recite Kiddush in the Sukkah before nightfall?
- It's pouring and it's obvious that it's going to continue all night. The Rama (639:5) says that you should make kiddush in the sukkah, so that your שהחינו will apply to it, and eat a kezayit there; the Mishna Berura says that you don't make a לישב בסוכה because many Rishonim say that you are not yotzei. Can you do this early, go inside and eat most of your meal, and then go back to the sukkah and eat another kezayit of bread?
- It's after sunset and not raining now, but it's going to start soon, before tzeit, and continue all night. If you wait, you won't have any chance to fulfill the mitzva according to the opinions that rain ruins the sukkah on the first night. Also, you won't be able to make a לישב בסוכה in any case. Should you make kiddush and eat a kezayit in the sukkah now, to at least get a chance of doing the mitzva according to those opinions, and then eat another kezayit after tzeit, to get 100% chance according to the others?