Past events in the calendar should be shown partially transparent (ex: 50 or 70% opacity)
This is a slight UI improvement suggestion that could make the UX a lot better in the calendar's month view, but also week view.
Past events are very much less relevant than current and future events. By setting their opacity to 70 (if not 50) percent, having a visual distinction with semi-transparent past events would allow the eye to instantly filter down to the most relevant information (and even spot where "today" is more quickly). It would be more efficient, and less visually overloading, to use Evolution's calendar with this way.
The only caveat you may have to watch out for is multi-day events that start in the past but are still ongoing today or until a future date, those should not be made transparent.
I believe this would benefit from being the default behavior (no option/preference required). Google Calendar, iCalendar and GNOME Calendar all do this by default.