Does the Jewish date change at sunset on Shabbat and holidays, or does it only change at nightfall, sometime after sunset?

? ב"ה Shalom, What concerns us on Shabbat is from when you are no longer allowed to do "melacha" =work on Shabbat, the date is less concerning. The times of Shabbat also have many laws how they are calculated , so I will try to make things simple and if you have a specific question you are trying to get at which I missed, you can send in another question. Shabbat officially starts at "Shkiah" sunset, from which time we are no longer allowed to do Melacha. But in addition to that, we have an additional mitzva of accepting Shabbat before sunset, "tosefet Shabbat" , from which time we are no longer allowed to do Melacha. However, women accept Shabbat even earlier by candle lighting and henceforth may no longer do Melacha. So Melacha may not be done even though "Saturday" has not started and it is still Friday for us. After sunset, there is the Halachic concept of "Bein Hashemashot" which is simply translated twilight, but we don't follow the astronomical definitions but rather the Halachic definitions. (See Mishna Brura hilchot Shabbat 331: 14 .) Bein Hashemashot is a time which possibly day and possibly night,and the very important ramifications in regard to what day it is arise, when a baby boy is born during Bein Hashemashot. If a baby born was born during Bein Hashemashot on the eve of Shabbat or during Bein Hashemashot just before Shabbat goes out, when will the Brit Mila,= the circumcision take place? The Brit Mila must take place on the 8th day. If the baby boy was born on Friday before sunset, the Brit will be Friday. If the baby boy was born on Friday eve after the stars have come out, the baby was born on Shabbat, so the Brit overrides Shabbat , and the Brit takes place on Shabbat. But if the baby is born during Bein Hashemashot, due to the doubtful status of Bein Hashemashot if it is day (Friday) or it is night (Shabbat), the Brit Mila is postponed until Sunday so as not to do Melacha on Shabbat. I hope I've answered your question. All the best