Season 16 ends on the 24th of February 1979, while Season 17 begins on the 1st of September. After almost 10 years the 27 week Season break has been accepted as the norm, to the point where most young viewers assume Doctor Who has always been broadcast in this way, and are completely unaware that prior to 1970 the show was on almost every single week of the year.
Season 17 is held in high regard by fandom for two main reasons - the introduction of Lalla Ward as Romana II, and one of the most popular stories of the classic series - "City of Death" (although it has to be born in mind that the unusually high viewing figures for Parts 3 and 4 were no doubt influenced by an ITV strike that reduced the number of TV channels to merely BBC 1 and 2).
For once the viewers appear to agree with fandom, and for the first 8 weeks the audience does not drop below 12 million. This could be due to the opening story seeing the first appearance of the Daleks for 5 years, and the first return of fondly-remembered arch-villain Davros.
Fandom does tend to see the rest of the Season as unremarkable though, and at first glance the viewers again seem to reflect this, with a drop back to around 8 or 9 million viewers for the remainder of the run. However this is again making the mistake of seeing a drop in audience as a failure of the show.
It's also worth remembering that when a season of Doctor Who is shown at this time of year it is competing with ITV's winter programming, which tends to be of higher quality than the rest of the year. The latter half of Season 17 still sees 11 of the 12 episodes drawing over 8 million viewers, while 9 of them draw over 9 million, and 3 peak at just over 10. Consequently the average for this season sees it just beating Season 14 to become the highest-rated season of Doctor Who since the show began. If BBC strike action hadn't curtailed the promised Douglas Adams-penned season-ender "Shada", those figures may have been higher.