That’s difficult to say. Most episode promotions end the Monday following the episode, but this is a bit bigger than the usual episode promotions. In fact, it seems more like the Winter update, which was launched on January 11 and ran through until January 28 (that update included Mr. Plow and some Simpsons snowmen and came with level 25).
To try to answer this question, I went into the files to look for an end date – and the only one I could find was May 29. Now, this could simply be a placeholder, or it could be the real end date. To me it makes sense as a real end date as it follows the month on / month off basis that Tapped Out events have kept to since September last year – and it also supports the educated guess I’ve made that the Squidport event will arrive at the end of May and continue through June (and before anyone asks, there’s a post coming on this – but type “seashore” into the search bar at the bottom of the page if you want to see what I’ve written on it so far).
If EA gives the usual episode-promotion “one-day left” warning, I’ll certainly let you know. But the short answer to the question that heads this post is that we can’t be sure at this stage when the Yard sale will end – it may be Monday, or it may be that it run through until as late as May 29.