Here’s a nifty post by Jonathan Snook where he walks us through how to make a calendar interface with CSS Grid and there’s a lot of tricks in here that are worth digging into a little bit more, particularly where Jonathan uses
grid-auto-flow: dense which will let Grid take the wheels of a design and try to fill up as much of the allotted space as possible.
As I was digging around, I found a post on Grid’s auto-placement algorithm by Ian Yates which kinda fleshes things out more succinctly. Might come in handy.
Oh, and we have an example of a Grid-based calendar in our ongoing collection of CSS Grid starter templates.