Paradox has put on a promo for Millennia, probably to promote the Atomic Ambitions DLC.
Right now it is possible to play Millennia for the next 3 days until November 18 for free.
If you decide after this free play time that you need more, then you get also a 50% discount (until November 26) on the base game (19.99 instead of 39.99) and a 30% discount on the premium edition (41.99 instead of 59.99).
The free access as well as the discounts are available on the Steam page here.