T0dd:
According to PMEG, and also according to my own sense of usage, LUNDE means "on Mondays" in general, not a particular Monday. PMEG has en (la) lundoj, en ĉiu lundo.

So, JE LUNDO or LUNDON means "On Monday", where a specific Monday is understood. The use of the -N form in the second case is simply an example of -N replacing the preposition JE.

It's interesting that Bertilow uses the form EN LUNDOJ or EN ĈIU LUNDO in PMEG. We do use EN commonly with months and years. Although EN is a spatial preposition, we metaphorically conceive of months and years as spaces, within which events are located, so EN works. I don't think I've heard EN used with specific days very much, if at all, but I see no reason it couldn't be used that way. If JANUARO is a kind of space, within which things can happen, why not LUNDO? Indeed, why does Bertilow use EN ĈIU LUNDO instead of JE ĈIU LUNDO as a definition of LUNDE?

That's interesting, thank you for the information. You and Bertilow are certainly right, it's more precise to use lundon or je lundo for this meaning.
In my dictionary from Erich-Dieter_Krause, who is also a member of the Akademio de Esperanto (like Bertilow), both meanings are mentioned for lunde.

A note on use of "sur", I believe that there is precedent for saying "sur televido", and not necessarily in the sense of a potted plant sitting on top of your TV set.

I personally prefer 'en la televido' but there are plenty of examples in Esperanto literature of uses like "sur la bildo", "sur la foto", etc.

Not to mention that we very commonly say "sur la vizaĝo" or "sur la muro", when we don't mean on *top* of the wall, but on the surface of the wall or the face.