Many progfans I know, usually listen to this band's music, but they rarely admit they do. Styx are between prog (or art) rock and plain pop, nonetheless they wrote many excellent songs I could easily put in my blog. That's the case with this "A Day", full of soul and bluesy influences, but also featuring a clever architecture, including starting with beautiful instrumental passages and a lot of liquid sounds in the first, down tempo part. Then, the mood changes and the tempo rises up, with an excellent electric guitar solo well supported by the rythm section.
"Styx II" is probably one of the "proggest" albums by Styx.
Some keyboards take the foreground scene at minute 5:49 and show up a brief, devilish progression. The sung theme reprise with a little more pop rock arrangement ends up a track that's definitely worth a progger's attention. That said, I also like the easier side of the band, but songs like "A Day" officially label them as a prog related band. IMHO, of course.