MySQL - Store date as a value with an ID?
It sounds like you have reinvented "Entity-Attribute-Value" schema.  This will be painful in the long run.  Meanwhile...

Do not "normalize" dates or other 'continuous' values (such as FLOAT).

The DATE datatype is only 3 bytes, and may as well be put in whatever table it best fits in, not normalized into an attribute table.

Relationship tables usually do not need an AUTO_INCREMENT id PRIMARY KEY since a combination of columns makes a better PK.  More tips here.