Littman asked:
If it is possible to prepare three days before Shabbos why not allow marriage on Sunday?
The Kollel replies:
The reason of Shakdu alone was indeed not sufficient to make a Takanah, since it was possible to prepare before Shabbos.
However, after Ezra instituted that Batei Dinim sit on Mon. and Thurs., it became necessary to institute a Takanah allowing marriage on only Sun. or Wed., because of Ta'anas Besulim. Chazal chose Wednesday over Sunday for the reason of Shakdu.
The Hafla'ah adds that if the three days are immediately prior to the wedding there is less chance of him making a false accusation since he had just worked so hard. (See also Re'ah, quoting his brother Rabeinu Pinchas.)
Dov Zupnik