This was the full name of a band from 1967 to 1974. It's a Beautiful Day was primarily the work of David LaFlamme, a former member of the Utah symphony. His specialty was a 5-string violin. He and his wife, Linda, and the rest of the band put out two albums. The first was self-titled and had a very non-60's cover: A girl in a white dress looking from a mountaintop, looking like a poster from The Sound of Music more than anything else. The song remembered most from this album, if you can find anyone who remembers it at all, is "White Bird." Other folks found "Warm Summer Day" a good tune, as well.
These were, for the most part, long songs, sort of reminiscent of what the Grateful Dead was doing at the time. In fact, LaFlamme was buddies with Jerry Garcia and was even auditioned for the Dead at one time. On their second, and last (as far as I know) album, Garcia sat in on two tracks. This album was titled Marrying Maiden and came out in 1970.