It's probably a holdover from the old rite. De anno et ejus partibus in the front of the old Missal has the following:My quick research suggests some confusion about weddings proscribed during Lent, which may be local custom, but which is not canonical per se.
N.B. "ordinary time" here means outside of Advent and Lent, not just the time after Epiphany and Pentecost. Although there may be local restrictions on nuptial solemnity, there appears to be no general prohibition against nuptial Masses during Lent in the new rite. In the old rite, the tract "Ecce sic benedicetur" (which is not in the OF chant books) is used from Septuagesima until Ash Wednesday, and also during Lent when allowed. If there is only one reading before the Gospel, I believe the Gradual alone could be used at an OF Nuptial Mass outside of Eastertide, since the Alleluia or Gospel Acclamation is optional.CONCERNING THE CELEBRATION OF MARRIAGES
Matrimony can be celebrated at any time of the year. Let the solemn blessing of marriages be forbidden only from the first Sunday of Advent to Christmas, inclusive, and from Ash Wednesday to Easter Sunday inclusive. The Ordinaries of places nevertheless can, during the aforementioned times and under sound liturgical laws, permit it as a substitute for ordinary time under just cause, if the spouses are notified to abstain from too much pomp.
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