Interesting – I presume nowadays it's the other way around, so ITV tells the soaps when to have their big moments, such as during the week of the BGT finals.
Well, presumably it's give and take - ITV ask if the soaps can have something special for particular big moments in the schedule, and the soaps will approach ITV to let them know when they're planning something big, so you don't end up with a major episode being wasted (the obvious example being the Rovers burning down at 6.15pm during the World Cup). All the usual business of running a channel, I suppose.
Before I'm a Celebrity became fixed in its slot, around the turn of the century for a couple of years ITV would always have a big week in November when they would show Corrie and Emmerdale five nights a week and launch a few big new shows, seemingly just to give the channel a big boost at a very important time for advertising, a bit like Sweeps in America I suppose. The first time they did that was in 1999 when they also had that After Hours special at 10pm every night with the return of Bet. Presumably the scheduling of that was half Corrie having this big storyline lined up and half ITV wanting something special.
But to take it back to the general point, sometimes they can just latch onto any kind of anniversary as an excuse. We didn't see it on BBC4 but the 900th episode of Top of the Pops in 1981 was a special episode where they launched Yellow Pearl and the new titles and so on. Of course, nobody normally celebrates 900 episodes of anything but seemingly Michael Hurll wanted to do a relaunch and so decided to hang it off an arbitrary anniversary to make a big deal of it to get a few column inches. And so the 35th anniversary may have had more to do with wanting to get something quite big on ITV in an important time for adverts than a particular desire to celebrate the anniversary. It's always helpful Corrie anniversaries are always in the run-up to Christmas.