According to the Canadian music business journal RPM Weekly dated 15 September 1979:

“In further support of the group, CBC-TV and a U.S. television network will be airing a special on the group, titled ABBA In Switzerland, September 27. Clips from the special were shown last week on the Jerry Lewis Telethon, along with pre-taped messages, and will also be broadcast across Canada on the The Raes' Sept. 11 show, in advance of the tour.”

The main thing of interest here is the implication that there was a pre-taped message from ABBA on the Jerry Lewis Telethon. According to IMDB, the broadcast began 3 September 1979.

"Jerry Lewis MDA Labor Day Telethon" Episode dated 3 September 1979 (TV Episode 1979) - IMDb

But since it was a 21-hour broadcast it went on until 4 September 1979, and, according to an article in Aftonbladet, ABBA’s promo clip for Does Your Mother Know was shown in the early hours of 4 September 1979 (NB: not a clip from the Switzerland special, as per RPM Weekly ).

Thanks to Carl Magnus Palm.