Poor contrast with dark theme active
gnome-calendar v 3.28.2 on Ubuntu 18.04.01
Using Adwaita-dark theme the whole-day events are not readable with almost any calendar background color. See the difference between whole-day and standard events at day 23 in the attachment.
If a light theme is active (e.g. Adwaita), the events are readable. Since for whole-day events the text is always shown on top of a colored box, I think the non-dark text color should be used anyway for them.