I actually got into Green Carnation well after Light of Day, Day of Darkness. I didn't know anything about the band or that they had an album like that. I started with The Quiet Offspring, which is very different from their other stuff. I was kinda underwhelmed at first, didn't really like the singer, etc.
But something stayed with me enough to keep it and it really grew on me. Also, I saw them live at a small bar around that time and they were really good.
So I went back and discovered Light of Day, Day of Darkness. It needed some growing time too, but for me there's definitely something special about it. So, I will agree with jingle that you should check that album out just because it's so good. And I also agree with him about the lull in the middle - sometimes I listen to the whole thing and sometimes I skip through it.
However, I doubt they'll be playing it at ProgPower next year. So don't use it as your ProgPower prep. On their recent live stream, they did a mix of all of the rest of their albums, so it's hard to say where you should start. However, they don't have that many, so I say just listen to them all.
- Edit: you can probably skip their first album as far as ProgPower prep goes; it doesn't have their current singer and the redid one of those songs anyway for their latest album.
I have a soft spot for The Quiet Offspring because it was my first (and hopefully ariich likes the songs I sent him from that album). I revisited it earlier this year and remembered again how much I like it and have been listening to it, and the band in general, a lot lately. I couldn't believe it when they were announced for ProgPower - perfect timing for me and I can't wait to see them again.
I'll add one, or two, more things. I don't know why, but their singer is way way way more impressive live than he is on the albums. At least for me. And second, you've probably read people raving about their 2016 set. Deservedly so. I can't even explain how good it was. But I'll caution against measuring them against that set. Both for those like me who saw it, and those who didn't. The setlist will obviously be different, and I imagine there will be a different vibe to their set next year than there was to that one. No need to hype it up in your mind so much that the real thing can only be a bit of a let down.