I use a template [1] that has the dates (including week numbers) there already. That way, I only need a text editor.
In my work, similar events happen on the same week numbers in different years. The importance of week numbers probably depends on your area of work.
For me, one thing is a day. That's also the context I need when reviewing and planning. But if I remember correctly, previous HN thread on calendar.txt had a suggestion where someone had an event-per-line format, if you prefer that.
[1] https://terokarvinen.com/2021/calendar-txt/calendar-txt-unti...
Also: claims to be "one thing per line", yet allows and encourages multiple events on one line (as long as they're on the same day). This is a calendar - is an event not the main "thing" we're dealing with?!