I think "wichtiger" is the comparative form, so why do we use it here, even though we're not making a comparison? Is it because there's some implicit comparison here in German ("today is a more important day than other days")?
Edit: just found out "today is a more important day than other days" actually uses wichtigerer, so I'm even more confused.
Edit 2: discovered the same applies to schöner and guter, and probably more, so there's something here I'm not aware of.