Wait, what? Yep, it's true. Here's the explanation.
The readings in the Lectionary are snippets from the Sacred Scripture. These lections are grouped by themes, and therefore not read in the same order as they are found in the Bible. When you take selections from the Bible and read them separately, many of them will seem out of context. When this happens in the liturgy, the Church adds an introductory phrase called an incipit at the beginning of the reading to make it easier to understand.
The reading from the Lectionary at Mass may start with the phrase in those days, however you won't find it in the actual chapter and verse. The only times the phrase actually appears in the Bible (that I can find) are Joel 3:2, Matthew 24:36, and Luke 2:11.
Historians suspect that incipits were initially used by the readers orally to introduce the passage, then were written in with the readings over time as lectionaries were assembled.
There are times when an incipit is actually part of the Biblical text, such as In the beginning which introduces both Genesis and John's Gospel. It was popular to reflect these introductory phrases very artistically in historic versions of the Bible, especially with decorative and distinctive incipit pages proceeding the rest of the manuscript.
Paragraph 124 of the General Introduction of the Lectionary for Mass (GILM) talks about the incipits that were inserted into the Lectionary:
In this Order of Readings the first element of the incipit is the customary introductory phrase: "At that time," "In those days," "Brothers and Sisters," "Beloved," "Dearly Beloved," "Dearest Brothers and Sisters," or "Thus says the Lord," "Thus says the Lord God." These words are not given when the text itself provides sufficient indication of the time or the persons involved or where such phrases would not fit in with the very nature of the text. For the individual languages, such phrases may be changed or omitted by decree of the competent Authorities.So maybe you've never noticed the Lectionary incipits during Mass before, but I bet you will now!