Why?
Actually I was going to eventually buy the game. Is that bad idea?

There is no win situation here. If the discounts are low, or lower than previous sales, there are complaints, if the discounts are high, people complain, if we were payed to receive the game there would also be complaints i bet. The deal is probably meant as publicity to draw some atention to the devs new game.

Possibly spring sale tomorrow?

It's already known that there will be a sale tomorrow. Here's one thread dealing with it, and there was also one where somebody linked a German site which jumped the gun and published an article about the sale's start, though I can't find it right now.

I hope the wishlist sale start in next 5 hours.

Is it not supposed to start on Monday?

A little bird (called MaGog) told me that it's "Bard's Tale, The" at -80%, $1.99 (hard to discover otherwise, as GOG seems to have forgotten to post it in the Special Offers box).

Thank you.
But GOG often have hidden sales, some of them are concurrent with Daily Deals.
Since GOG does not announce it, we have to wait for 24 hours and see if the discount ended.

If The Bard's Tale only have the discount for exactly 24 hours (+- 1%), then it could be an Daily Deal.

It is, they announced it on GOG Twitter.

Hey, I like the last Daily Deals.
I've bought We are the Dwarves yesterday and today I've decided to take Bard's Tale also (however I'm planning to check classic BT1 and BT2 first;).

There is also a tweet on GOG's Twitter. I will add it to the list anyway.
They have managed to tweet about it, yet have not added it to the salebox? Come on GOG!!

EDIT: Also thank you mrkgnao. :)
It is a daily deal. They have a tweet announcing it.

Bard's Tale is indeed daily deal.

Two official Daily Deal promo cards appear at homepage just before the promo ends...
Nice job GOG!

Hope the wishlist sale today will be less buggy.

Two cards?
I am just seeing The Bard's Tale's card in the salebox.