I suppose I like the idea there are jokes still to be discovered!
Absolutely! And it's always nice to newly notice background details, whether they're humorously and narratively significant or not.
And thanks to Levent for starting off this interesting thread and these trains of thought. Having been inspired to re-read Picaros last night, and paying attention to the way the dates of the adventure are specified by that Carnaval poster they drive past on the way to the countryside villa, I had the sudden realisation that we can actually establish the year from this, because most traditional Latin American/South American carnivals end at the start of Lent – specifically at midday on Ash Wednesday, I discovered on looking that up – and because the date of Ash Wednesday is quite different each year, moving over a range of some weeks in relation to the similarly moveable date of Easter, of course.
The poster tells us that the Carnaval dates are 22, 23 & 24 February, and I've found that the only year between the late 1960s and the late 1970s on which Ash Wednesday fell on Feb 24 was 1971 (with most other years in the likely period for this book not even being very close).
1971 seems to works OK in terms of details like the colour TV transmission (assuming Belgium introduced that at the start of that year rather than midway through!), the Boeing 747 they fly home in (which according to Wikipedia entered service in 1970) and clothing fashions such as the outfits of the members of the Dripping Tap pop group. But I'm not suggesting that this Ash Wednesday date detail necessarily means that Hergé particularly intended it to be specifically dated to 1971, and thus to be read as something that occurred four or five years in the past by the time the story was published in the mid-seventies. But perhaps he was first sketching that Carnaval poster in 1971 and looked up the date of Ash Wednesday for that year to use as a realistic and reasonably typical sort of date.
Or maybe it was just random! Time and chronology in the world of the Tintin books seem to rather flexible compared to that of the real world.
(Or maybe General Tapioca doesn't approve of the Carnaval dates shifting around annually and has pinned them down to the same dates each year since taking power!)