They say those are the official days but they really aren't. Prior to the sudden leave of players due to the 90cc transfers - solo ranked typically happened everyday but Imp dominated. You'll find that most of the time Pub Side PvPers are dubious/refuse to queue solo ranked and that's been the case since S3. But I could be wrong and Pub Side PvPers just queue for solo ranked when all of the Imperial ones are offline.