When is the best time to buy equipment??

Around 3PM Pacific time.
The CES show in Vegas in early winter is when a lot of manufacturers are showcasing their upcoming products. These new products typically come out either in the spring, when people's wallets have recovered from Christmas, or in the fall, in anticipation of the upcoming holiday buying season. Therefore, with a lot of new product coming out in the spring and fall, it would seem logical that dealers would like to get rid of the old stock before the latest and greatest new thing comes out. For this reason, I think that early spring and late summer should be the best time. Think of how car dealers discount their products just as the new model years are released.

I don't have any real facts to back this up in terms of sales figures but it seems to make sense to me, at least for new products. The second hand market might lag the new product market slightly as people get the upgrade itch after they've had time to digest the new product offerings.

It's an interesting question you pose. It would also be interesting if AudiogoN gave some statistics on a month by month basis as to the number of transactions the site has facilitated. However, AudiogoN might not know how many transactions resulted from the ads placed here. Also, they might not want to release such information as it could be confidential for business reasons.
whenever my wallet is fat.
April may be one of the worst times, because people have more disposable income from tax returns and work bonuses, so gear moves quicker and sellers are less motivated to sell at a discount. Based upon my selling experiences, I'd say December & January are the best times, as people are putting their disposable income towards paying off Christmas bills, resulting in sellers being more likely to accept lower prices for slow moving gear.
Look over the classifieds On mondays, people come to their senses then. I think over the summer you will start to see a flood of amplifiers ect. Right now on agon the prices are up from a while ago, Just wait and dont make snap decisions. Most of the time your the only buyer anyway :)
I said April because I've seen so many ads by people looking for cash to pay the taxman!!! And these people usually file at the last minute. People GETTING money back from taxes usually file as early as possible to get the cash.
Just as soon as the interest-only mortgages are coming to term.
I did a one person survey, that person being me. I was just curious and this is what I came up with:

Feb was by far the busiest month (30) while June was the least active (3). Jan and Oct were tied for next busiest w/14 ea. while April and July were the next least active with 7 ea. The other 6 months were all close with between 8 and 12 ea.

I was off by one somewhere but that's not enough to skew the numbers.
There never really is a good time, but when I get that uncontrollable stirring in my loins I start cruising the Audiogon Boulevard looking for some audio fun.
During the wintertime, when outdoor activities are severly limited, is when most folks buy gear. Many people like to plan vacations in the summertime & are more worried about other family things. As such, the summertime is the best time to get good deals on A'gon when sales slow down.
Any time Mercury ain't retrograde hehe.

When the wife is having a "weak moment" or has her arm in a sling.
Either works well for me.
The absolute best time to buy equipment is when you don't REALLY HAVE TO HAVE IT AT THAT MOMENT, but you come across that screamin' deal nonetheless, and you buy!
Best time is when you exactly find what you are looking for.

Waiting for a better time : You will save a few bucks, but the extra work involved may not be worth it, and you may not get exactly what you are looking for.

So, this post starts and ends with the same issue : Try and get exactly what you are looking for ! This might be a consolation if you find out later that you have paid too much !