What What is the significance of 'dawn-break' here?
Seforno: The punishment had to take place with sunrise, because they worshipped the sun. 1 So when the sun appeared, Hashem struck.
Rashbam: The Pasuk mentions dawn-break to demonstrate how patient the angels were with Lot, who arrived in Tzo'ar only with sunrise. They waited for him 2 due to their love of Avraham Avinu.
Seforno: As the Gemara says in Berachos (7a) ' ... when the sun rises and all the kings of the east and the west remove their crowns from their heads and prostrate themselves before the sun', thereby asrousing G-d's anger. Refer also to 19:15:2:1 and note.
It is not clear however, how the Rashbam then translates "Vaya'itzu", which all the other commentaries translate as 'and they urged'.
Hashem destroyed them "u'Chemo ha'Shachar Alah" - What time is the Pasuk referring to?
Rashi (verse 24): It is dawnbreak, when both the sun and the moon are in the sky simultaneously. 1 Because some of the people worshipped the sun and, some, the moon, 2 had it been at another time, some could have claimed that had their god been in the sky, he would not have allowed S'dom to be destroyed.
Moshav Zekenim: It is not dawn. If so, it would have said "uch'Alos ha'Shachar"! Rather, it was a bit of light, like dawn.