So I hit it in the outline and go AAAARRRRRGGGHHHH! I need to world-build their wedding customs!
Toyed with vows but no, I want something different. (Even if it gives some readers conniption fits because they are unaware of the possibilities.)
But it is a kinda a copy of Christianity, put through some fictional wringers. So I skipped lightly over many of the possibilities to snatch the Dark Age standard: you have the marriage contract, and that makes them espoused. Then there was the tradition of showing them to their bed in a rather rowdy and bawdy ceremony (which bishops sometimes prohibited priests from attending), which would need rather toning down to a more sedate ceremony, but the contract would work fine. Among other things, it let me move briskly through an event necessary more for its importance to the characters than the plot. (The fact, not the ceremony, is the turning point.)
Even had the heroine pondering how simple the contract has to be for the two of them, with no lands to apportion.
Then I started to wonder whether to lift from later in the Dark Ages to have them get a blessing at the chapel between the contract and the wedding feast. Hmm. Decisions, decisions.