Since you say Angelus Press then you’re obviously talking about the Extraordinary Form of the Mass/Traditional Latin Mass.
My understanding is that the “default” for a weekday with nothing on the calendar is to celebrate the ferial Mass - meaning essentially the Mass of the previous Sunday, including the readings, minus the Gloria and Credo (the Gloria is retained in Paschaltide). Celebrating the feria seems to be fairly common in Advent, Christmastide, Lent, and Paschaltide. Lent, at least, actually follows different rules, and even in the Extraordinary Form has propers for each and every ferial day, and I think those are normally mostly observed. Octaves and the Ember Days have propers or special rules, too.
At least in the FSSP community here, they hardly ever celebrate ferial Masses in the seasons after Epiphany and after Pentecost - “green” weekdays (known as Ordinary Time in the Ordinary Form). Instead any weekday that isn’t some sort of feast is generally celebrated as either one of the Votive Masses or a Requiem Mass. So you would need to consult the bulletin (or the priest) to find out what Mass is being celebrated on a given day that’s not in the Proper of Seasons or Saints. (For that matter, on any “4th class feast” also known as “commemoration” they also may be celebrating a votive Mass.)
For those who don’t know, in liturgical terms, feria pretty much means a weekday - most any day that’s not a Sunday or a Feast. So a ferial Mass or the ferial Office is just the Mass or Office of the weekday non-feast. Basically every day is either a Sunday, a Feast, or a feria. (A votive Mass is by definition not the Mass of the day, feast or feria.)
Edit: I should clarify that on a free Saturday the Mass and Office of Our Lady on Saturday is the default. Also, everything I have said applies to the Extraordinary Form, since the Ordinary Form has different rules (including proper readings for every single day of the year).
