I suspect that, for the most part, discount listing days aren’t very predictable.
It’s been a while since I watched total listing numbers but the discount listing days seem to correlate most strongly to a precipitous drop, in the days immediately preceding a discount, to the total number of eBay auction listings. As such it appears as if the decisions to hold a discount listing day are impromptu and made quite abruptly.
You can find ebay auction count charts here: http://www.quotetracker.com/cgi-bin/cal.exe?EIND though the relevant daily and weekly charts don’t seem to be working for me today. Watching them won’t give you much warning. The listing numbers always seem to (used to) drop somewhat on Mondays (Monday discounts may be a new trend.)
Having stated this there still may be some patterns available to a discerning technical analyst. I notice, for example, that every year, from 2003 on, there appears to a spike in listings close to Valentine’s day that could correspond to discount listing days. It could be that sellers, confused and smitten by the fervent ardor, and passionate fear (the hell to pay if you forget) of the week allow listings to lapse and this is subsequently followed up by a discount listing day to help them regain their senses.
Marty