What date to place the patronal feast day of St. Mary’s Church? According to Catholicism, what are the norms or rubrics that explain what is the patronal feast day of St. Mary’s Catholic Church, when no particular mystery or other title has been indicated?

In my diocese alone there are many churches dedicated to the Virgin Mary with no other indication as to know when we should celebrate the parish feast day!

I am interested in the norms and rubrics for both the Ordinary and Extraordinary Rites of the Mass, whether they are different or the same.
Your hunch "that it is the Feast of the Assumption" appears correct, at least according to


Rev. W. J. Wiseman, “The Titular Feast,” The Pastor 3, no. 5 (March 1885): 129–36.


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.