IIRC, the weekend sales usually show up on Thursday and end early Monday*, and the midweek sales show up on Tuesday and end on Friday (not sure what time, but they're showing Friday for me now), bearing in mind that I'm in the USA and on the opposite side of the world early Monday might be late Tuesday. I think. I'm not 100% certain whether midweek sales show up on Tuesday, or on Monday sometime.
They also frequently have shorter sales (I see several sales right now with 39 hours left, which I don't recall being on sale yesterday, so I'm assuming they're either 48h sales or my memory sucks), and rarely have week-long sales (and I don't remember what day they start and end those), and of course they also have sales that don't fit any of those parameters and just run from some date to some other date (and pre-order sales).
I think they usually prefer to keep the sales short - most of them seem to be, or at least most of the well-publicized ones are.
I'm also not sure of the precise times, since I tend to just glance at what's on sale, and then never buy anything, but the last time I was actually paying attention to the times, I think the weekend deals vanished around 10 am US eastern time. If my memory is accurate. Maybe someone else has been watching them too.
So anyways, if you wanted to encourage buying games on sale you'd probably have to time it for one or the other: either the weekly or the weekend sales, since they don't really overlap in any useful way (even if they did, one would be starting as the other was ending).
For example, for the midweek sales, entries on Tuesday and Wednesday, voting at any time, and the drawing on Thursday before the sales end so that you don't have to try to beat the ending in the morning (if you're in Australia or thereabouts the time considerations are probably different for you).
It'd definitely be much faster-paced, though, which might be a downside.