I did this on cnnfan.org but may be off interest here. A comparison of the CNN/US ratings every hour between 5pm and MidnightET in the 25-54 demo for last Friday-Monday.
5pm - Situation Room weekdays/CNN Newsrrom weekends
Fri - 121
Sat - 175
Sun - 39 (ouch)
Mon - 140 (best MSNBC though)
6pm - Situation Room weekdays/Best of Situation Room Sat/CNN Newsroom Sun
Fri - 134
Sat - 137
Sun - 55
Mon - 123
7pm - John King USA weekdays/CNN Newsroom weekends
Fri - 75
Sat - 133
Sun - 92
Mon - 131
8pm - Parker Spitzer weekdays/documentaries weekends
Fri - 70
Sat - 198 (repeat documentary)
Sun - 238 (new documentary)
Mon - 150 (good night for Parker Spitzer)
9pm - Piers Morgan Tonight (new weekdays/repeat weekends)
Fri - 121
Sat - 168
Sun - 193
Mon - 208
10pm - AC360 weekdays, CNN Newsroom weekends
Fri - 135
Sat - 124
Sun - 153
Mon - 193 (3rd place, but kept most of Piers Morgan lead in)
11pm - AC360 repeat weekdays, 8pm documentary repeat weekends
Fri - 148 (beat 10pm)
Sat - 136
Sun - 232
Mon - 106
Midnight - Piers Morgan repeat
Fri - 173 (Piers Morgan got a better repeat rating than the first airing)
Sat - 142
Sun - 152
Mon - 88
So the best ratings at 5/6/7pmET were on Sat, at 8/11 on Sun, at 9/10 on Mon, and at MidnightET on Fri
A fairly typical reflection of CNN's ratings there. Weekends perform the same as, or even better than weekdays. This is down to CNN's Documentaries (see the boost they got from 8pm onwards Sunday) and weaker competition. MSNBC show non-news programming most of the weekend, and HLN is mostly repeats. Only Fox and CNN have new news programming in weekend primetime. While Fox had a bad weekend, CNN beat them in primetime in the demo both Sat and Sun
Another thing to point out here is Piers Morgan's midnightET repeat ratings. They were very strong most of last week, something which was hardly noticed. While 9pm is the most important, a strong midnight showing obviously boosts CNN's overall total day figures
Doing well at midnight, or the weekend clearly does not impact on the big 8/9/10pmET weekday slots, but there are positives from CNN's ratings. The big thing obviously is to see what level Piers Morgan settles at in his 9pm slot, and to see how much that boosts AC360 at 10pm
(All info taken from TV Newser)