I know people don't push off weddings, I'm asking what is the source?
At least for chabad sources
Once the date for a wedding has been fixed, the custom is that it is not postponed (and certainly if there is no valid reason to do so).
אגרות קודש ח"י ע' יב
.
If for whatever reason both mechutanim agreed to postpone the date of a wedding, then on the day that was originally fixed as the wedding day, they should write Tenaim (in addition to the fact that they had already made a Vort) and the parents from both sides should get together and discuss matters pertaining to the forthcoming wedding. This meeting should be in the form of a simchah gathering with a Seudas Mitzvah, such as a Siyum Masechta or a Melave Malka, and they should farbreng — with simchah.
אגרות קודש ח"ז ע' לח