There are a number things that sellers say; "I can list on any day of the week and get my sales" or "Never list on a Monday or Tuesday". In truth, when, what your looking for goes on auction doesn't matter. What does matter is when does it end?
If you can't get to a computer and bid when the auction ends, it really doesn't matter how much you want it. The alternative is a snipe program or service (or you blond sister) but that's covered elsewhere I'm sure. It also matters what your looking for. The rare and exotic don't show up much on eBay any longer because people have found how to either sell it for more or buyers have already gotten there before they could list it. Don't think your going to get that 1 ct Victorian old mine cut ring for a song, there will be at least 10 people looking for items just like that to put in their real brick and mortar stores on any given day.
But for the regular items like tools, electronics, and DVD's, try the big summer weekends. The 4th of July and Labor Day weekends cut activity on eBay a lot. There are also a lot less potential bidders during the summer months when the regular people who have regular jobs and children have to cut back on eBay shopping because of children being out of school which means sitters, summer camps, and vacations and the extra money that takes.
You can try the other holidays of the year but figure that come evening (except for St Paddy's Day, 5th of May, and Halloween), most everyone is back checking on the status of auctions they are watching. Easter is only good the first couple hours of daylight when the auction ends. After noon, it's a regular bidding Sunday.
Some people will have access even when they are supposed to be working, it happens and the employer looses, and you might too.
I've found that waiting for the last few seconds does work, it also keeps you from running up against the hopelessly addicted to bidding. Those with more money than sense, who keep bidding until they get on top, only to go back and go through the process all over again when they are topped by another like personality (even though there are 15 other identical items on eBay at the very same time, they stop at the first or second one they find and battle it out, much to the seller's delight!). There is always someone out there with more surplus money than you and they can't be bothered to see if the local super department store carries it for $7.99 and wins the auction for 10.28 plus the $5.99 shipping.
Winning isn't about how much you can spend, it's how well you shop, research, and when you place your bid.