Welcome to Namepros. Are you the 'Scott' on that domain's current lander?
Very nice domain. Registered since 1997. Are you the original registrant or did you buy this later? I could check for sales but it's late and I'm tired
Perhaps you're one of those lucky ones who regged a great domain early, used it for biz or personal reasons all this time, kept totally out of the loop about domain names, and now you're surprised to find out that a reg fee domain 25 years ago could now be worth thousands or millions $$?
Anyway:
According to dotdb, coolday is taken in 32 extensions and is part of around 60 other domain strings/extensions. Very desirable.
Also, a LOT of trademarks, both for one word Coolday and two words Cool Day. Maybe one of those TMs is yours? - Again, late, I'm not gonna check.
Just a cool term. And versatile, could mean a lot of things to a lot of industries. A reseller price would be somewhere in the mid four figures easily, 3K - 6K range depending on which marketplace. Maybe more.
End user price, start at 20K (I agree entirely with
@enterscope but I feel that is an easy minimum/conservative end user price). On a good day, closer to 40 - 60K range. On a great day, an end user with big budget might even break six figures. I would say that is within the realm of reality, but very highly optimistic. I broker premium domains; I feel strongly your sweet spot for this one would be around 40K.
***Valuations given do not take into account stuff like traffic, backlinks, etc etc, which if substantial can add a lot to a domain's value. I just appraised it as a domain only.
Good luck. I like this one a lot.