The timeline below shows the total seconds of airtime across CNN, MSNBC and Fox News since the start of last year in which "Ad Council" was mentioned somewhere in the onscreen text. Note that this records only the seconds where Ad Council is mentioned, which is typically the last few seconds of each ad, meaning the actual seconds of airtime of Ad Council advertisements is much larger.
The large surge on MSNBC on December 10, 2020 is an interview on "The 11th Hour With Brian Williams" about how the ad council would be working to promote Covid-19 vaccination. Other notable increases are from late March to late June 2020 and since mid-January 2021 as vaccinations have picked up.
The graph below shows that Ad Council ads have run nearly equally across the three channels. (Remember again that these airtime counts are just the last few seconds of each ad so the actual ad airtime is much higher).
Hopefully this inspires you on your own adventures to examine the patterns of television advertising!