http://www.saintsfc.co.uk/page/NewsDetail/0,,10280~2433451,00.html "Due to the extenuating circumstances beyond our control, the Saints vs Birmingham clash which was originally due to be screened live on Sky Sports on Saturday 17th September at 5.20pm, will now take place at 3pm on Sunday 18th September instead, without live television coverage."
I think this is due to Birmingham playing midweek in the Europa League. They seem to be playing Sunday every week now.