The Rebbe writes:1
“The wedding day in general and the time of the actual wedding and standing under the chuppah in particular, are extremelypropitious moments; a time of segulah (Divine benevolence) Above. Bride and groom should therefore prepare thoroughly for that great day in their lives. They should jealously guard their time and increase their gifts of tzedakah, prayer and recitation of Tehillim.
“It is customary to fast on the day of the wedding and be meticulous to think only pure, G‑d-fearing thoughts while standing under the chuppah.”