Coming from a different culture myself, I wasn't aware of concept of "Advent Calendar" until 2014 when the first QEMU Advent came out.
Think of the term "advent" as "the arrival of a notable person or thing" — in this case, it's a thing :-)
That said, I don't see any issue in using that term here. Even if one is not religious (I'm not), they might appreciate the cultural aspects of it. Furthermore, the Call for Images[1] clearly says this in the postscript:
"QEMU Advent Calendar is a secular calendar (not religious). The idea is to create a fun experience for the QEMU community which can be shared with everyone. You don’t need to celebrate Christmas or another religious festival to participate!"
Think of the term "advent" as "the arrival of a notable person or thing" — in this case, it's a thing :-)
That said, I don't see any issue in using that term here. Even if one is not religious (I'm not), they might appreciate the cultural aspects of it. Furthermore, the Call for Images[1] clearly says this in the postscript:
"QEMU Advent Calendar is a secular calendar (not religious). The idea is to create a fun experience for the QEMU community which can be shared with everyone. You don’t need to celebrate Christmas or another religious festival to participate!"
[1] https://www.qemu.org/2020/11/26/qemu-advent-announce/