If you want to follow UNIX philosophy, why don't you write an augmenter/converter tool `caug` that adds "computed" information such as week number, weekday or even relative date?
> cat calendar-src.txt
2025
====
03-01 9-12 project groups
> cat calendar-src.txt | caug
2025-03-01 w09 09:00-12:00 project groups +tomorrow
+thisweek
> cat calendar-src.txt | caug | grep "thisweek"
2025-03-01 w09 09:00-12:00 project groups +tomorrow
+thisweek