This time of year there is often much made of the fact that the Sundays during the season of Lent are properly spoken of as being "in" Lent and not "of" Lent. This is because there are 46 days between Ash Wednesday and Easter, so the six Sundays in Lent do not count toward the 40 days of Lent. The question recently arose among some friends whether the Sundays of Advent are viewed similarly.
As I began to research this question, I could find nothing in Talley's Origins of the Liturgical Year, Reed's The Lutheran Liturgy, Lang's Ceremony and Celebration, Maschke's Gathered Guests, or in Lutheran Worship History and Practice which spoke of the Sundays during Advent being "in" but not "of" Advent. At the same time, many of these sources do make the note that the Sundays of Lent are not considered to be "of" the season, only occurring within it. Reed makes the comment, though, that "The four Sundays in Advent are all festivals," which would seem to indicate that the Sundays themselves not only bear the character of the season, but even create it.
Penitence has not at all times and in all places been considered characteristic of Advent as it has of Lent. The modern view by many that the character of Advent is anticipation and joy rather than penitence is not without historical precedent. Maschke notes that the first-known celebrations of Advent in fifth-century Gaul were considered a joyous festival with the use of white vestments.
In looking over the liturgical calendars of LW and LSB, there appear to be three ways to name Sundays which are related to a festival, without being the festival. Those are Sundays "in" a season, "after" a feast, or "of" the season. The only Sundays referenced as "of" the season are those Sundays "of" Easter. (In TLH, the only Sundays designated as "of" are those of Holy Week. Sundays following Easter are referred to as "after Easter.") This we know to be because Easter is considered to be an eight-week long feast, unlike other feasts which only cover a single Sunday. Those Sundays after Pentecost and Epiphany , are simply numbered to mark time following the feast and are not themselves part of it, and their precise numbering and propers have been variable even within the past century. This leaves the Sundays "in" a Season. Since they could not be "of" a festival such as Easter, nor are they "after" a festival like Pentecost or Epiphany, they are marked as "in" their respective season. While this preposition is noted as significant regarding Lent, none of the authors I researched made the same connection in Advent. Also, in LW and LSB, the Monday, Tuesday, and Wednesday before Easter are referred to as "in" Holy Week. Certainly these are still counted as a part of Holy Week and fully bear its Character even though they are designated as "in" it. It would appear logical to conclude that "in" is simply the most linguistically sensible preposition for Advent. Maschke and Reed's observations about Advent seem to indicate that the Sundays of Advent are not considered to be different in character than the season itself.
Also, because Advent is not set at a specific number of days, it is not necessary to exclude Sundays from the count in order to arrive at the appropriate 40 when beginning on a Wednesday. Advent flows from the number of Sundays preceding Christmas. It begins on a Sunday, and has a varying number of days, ending on a variable day of the week, while Lent begins on a Wednesday, lasts a specified number of days, and always ends on Sunday, and therefore revolves around the number of days in the season rather than the number of Sundays.
I would conclude that Advent's Sundays are labeled as "in" the season not so much because they are not "counted" toward the season, but more because the designation "of" is reserved for the special character possessed by the Sundays of Easter, and therefore not appropriate to Advent.
