If by “tomorrow” you mean Saturday, I can say that in the Latin Church, Saturday is traditionally the commemoration of the BVM. What the Novus Ordo prescribes I don’t know, but in the EF, in the absence of a specific feast, the Votive Mass of the BVM was/is often used on Saturday.
What the priest means is that he will use the Propers (Antiphons, Opening Prayer, etc.) from a Mass in honour of the Blessed Virgin Mary. But you can choose propers from another Mass - like a Mass in honour of another saint, or of all the saints, or for a mystery (like the Sacred Heart, or the Holy Eucharist), or for the dead, etc.
(To be honest, I’m not sure how exactly he is celebrating a Votive Mass since August 21 is the feast of St. Pius X- an obligatory memorial - which usually means that a Votive Mass is not said. I guess because he is traveling and so is using the permissions for the Missale Parvum. Or perhaps he belongs to a Marian order/congregation or he could be using the “real need or pastoral advantage” provision in the GIRM)
It ended up being the Mass from 16 Sept if I remember correctly. He loves the Blessed Virgin (I love that so many priests are so strongly devoted to her!) and wanted to have this particular Mass with us because he’d be gone by then and so would I. Priests are my favorite kind of people!!!
The ORDO for my diocese, Orange CA, notes it V2. =
Ritual Masses and Prayers for Various Needs and Occasions and Votive Masses, in cases of serious need or pastoral advantage, at the discretion of the rector of the church or the Priest celebrant (GRIM 376) are permitted.