It makes sense if you know you will always have exactly one record for that date. If there’s any circumstance or possible future where you might end up with multiple, then don’t. Also consider any implications of that for foreign keys referencing that table.

There’s also nothing wrong with using an integer or UUID PK as an additional field even on a table that has another field that would make a natural primary key.

Date as primary key is a pattern sometimes used in star schemas for OLAP databases, though separate y/m/d tables are more common afaik.