As explained in the marvelous Seder Nesuyin KeHilchatam (5:1):
"We do not arrange marriages for Shabbat from a safeguard decree that we
would come to write the Marriage Contract (Ketuvah) on Shabbat."
Also we do not enter the bride under the Chuppah (canopy) on Shabbat even
if the betrothal took place before Shabbat commenced because of the
"acquisition" that takes place (the acceptance of the ring is a legally
binding acquisition - we do not perform these legal niceties on Shabbat)
The sources for these are based on the Mishnah and Gemara in Beiya 36b -
37a, Shulchan Aruch - Orach Chayyim 339:4 and Mishnah Berurah 339:22. Also
Shulchan Aruch - Even HaEzer 64:5
One of the questions that I was asked when I was tested for Semicha was as
to the situation if notwithstanding a marriage had taken place on Shabbat.
Correct answer being that the marriage is upheld with no need for a repeat
May we blessed with many worthy houses being built in Israel!
A S Hill