I have not noticed any trends regarding more sales near the start or end of a month. But I have not analyzed the sales to see for certain if that is the case.
And yes, the sales fluctuate. But... well, look at the chart for yourself:

These are the sales of my product by month, since I started selling it March 15th of 2005.
What can you tell from this?
Well, sales of clearly picked up over time.
And you can see that sales are fluctuating less as time goes on.
And that since the start of 2006, only once have I sold less than 8 copies a month. And that in 2007 I sold at least 12 copies every month except one where I sold only 11.
You may also note that my sales in Feb aren't that bad. That's because I sold no copies the first week and then went nuts adjusting my advertiisng trying to find the problem, because I've never gone a week selling 0 copies. I also experimented with different budgets, and added new banner shapes. As a result I still sold a decent number of copies that month in the end, but I also spent more than I made on the software, so my profits for that month were effectively zero.
And don't be fooled by the low bar on the end for March. We're not halfway through the month yet, that's why that is where it is.
Now, you may be wondering about those spikes in sales. Are those around the same time? The answer is, no, those aren't even close to the same months, and it is not some annual fluctuation.
The first spike, in April 2005, was due to the software just being released. It was bought by several Blitz users, it was posted on Garagegames, I think I even posted a link on CGTalk.com.
The second spike in August 2006 was due to CGTextures.com opening up. They too posted on CGTalk and my ads ran there automatically due to a keyword targetted campaign I was running.
The spikes in May 2007 and Jan 2008, I don't know the origin of. I don't even rmemeber having a spike in May. I guess I'll try to use google and search on my product's name in those time periods if it allows me to do that, and see if I can find a mention of it somewhere. I may also have made a chnage to my advertising then which resulted in more sales but lower profits. I'd have to combine this chart with my google adswords statistics to determine my profit for each month.
If you want to see the sales for a specific year month by month just change the filename of this image. This is 2005:

Note that the individual charts for the years are not to the same scale.
Anyway, so as you can see, the sales do fluctuate, but not that much. Between about 12 and 16 copies a month right now. That's +-12% of the average.
And yes, the sales fluctuate. But... well, look at the chart for yourself:

These are the sales of my product by month, since I started selling it March 15th of 2005.
What can you tell from this?
Well, sales of clearly picked up over time.
And you can see that sales are fluctuating less as time goes on.
And that since the start of 2006, only once have I sold less than 8 copies a month. And that in 2007 I sold at least 12 copies every month except one where I sold only 11.
You may also note that my sales in Feb aren't that bad. That's because I sold no copies the first week and then went nuts adjusting my advertiisng trying to find the problem, because I've never gone a week selling 0 copies. I also experimented with different budgets, and added new banner shapes. As a result I still sold a decent number of copies that month in the end, but I also spent more than I made on the software, so my profits for that month were effectively zero.
And don't be fooled by the low bar on the end for March. We're not halfway through the month yet, that's why that is where it is.
Now, you may be wondering about those spikes in sales. Are those around the same time? The answer is, no, those aren't even close to the same months, and it is not some annual fluctuation.
The first spike, in April 2005, was due to the software just being released. It was bought by several Blitz users, it was posted on Garagegames, I think I even posted a link on CGTalk.com.
The second spike in August 2006 was due to CGTextures.com opening up. They too posted on CGTalk and my ads ran there automatically due to a keyword targetted campaign I was running.
The spikes in May 2007 and Jan 2008, I don't know the origin of. I don't even rmemeber having a spike in May. I guess I'll try to use google and search on my product's name in those time periods if it allows me to do that, and see if I can find a mention of it somewhere. I may also have made a chnage to my advertising then which resulted in more sales but lower profits. I'd have to combine this chart with my google adswords statistics to determine my profit for each month.
If you want to see the sales for a specific year month by month just change the filename of this image. This is 2005:

Note that the individual charts for the years are not to the same scale.
Anyway, so as you can see, the sales do fluctuate, but not that much. Between about 12 and 16 copies a month right now. That's +-12% of the average.