This is enforced more strictly in some parishes, I know, but the general notion is that weddings not be held in penitential seasons. Lent is considered a more seriously penitential season than Advent, by the way.

A pastor can judge the circumstances to be such that this rule not be observed--though the recommendation that weddings be planned far in advance makes such circumstances rare. For instance, if a military person were to be home on leave ONLY in a penitential season...

This is a law made by the Church, though, not by God and can be adjusted for circumstances. I know someone whose child was baptized because the grandmother would be in town during a penitential season--and not again for a long time. I know that's not your question, but the flexibility of a pastor to judge is part of this decision-making.