CORONATION STREET:
Aside from Christmas day episodes, Coronation Street had a "after hours" episode on a Saturday in November 1999, they did 6 late night (22:00) eps set in Brighton as a special week, but the Saturday ep was at 19:30 , got just under 12 million viewers, (and was the highest rated episode of the specials), at a time when regular episodes rated 16-18 million.
Of course on Millennium day, there was a one hour special edition, gaining over 15 million viewers.
Next up, at the time of the 40th Anniversary, they did a special week of regular episodes a week prior to their live ep, every night for a week in December 2000, whereby most episodes were in their regular timeslot, the Saturday episode went out after 9pm, and was 4million viewers less than the best rated episode of the week, down to about 12.5 million.
I do not watch EastEnders, but do recall a Saturday episode during the 1998 Football World Cup
Emmerdale has only had Saturday episodes when they fell on Christmas Day, such as in 1999, 2004, and 2010, but the first Christmas Day episode from Emmerdale did not happen until it had been on air for 25 years.