score:3
Your hunch "that it is the Feast of the Assumption" appears correct, at least according to
In the § "Practical Directions" (p. 131), he writes:
- Blessed Virgin is a general title. […]
(a) If the titulus is simply B. M. V., the titular feast is to be celebrated on the feast of the Assumption, where everything remains as now in the Ordo.
That's interesting it's the Assumption and not the Immaculate Conception (whose dogma Pius IX defined 31 years prior to this article, in Ineffabilis Deus). Of course the feast of the Assumption was celebrated even before Pius XII defined the dogma in Munificentissimus Deus in 1950, too. They're both great (1st class) feasts.
Upvote:-1
There are several Marian feasts, so it's hard to know.
Marian feasts:
(The dates are from the traditional calendar; you can see if they've been changed in the Novus Ordo calendar.)