There are such periods - such as the summer holidays - when people aren't buying as much. But I don't think it makes any difference. You might have your best run of sales when everybody around you is talking about how slow it is. Conversely, you might have a long dry spell while others are talking about their great sales.
So I would recommend not to think slow periods or bad. I make sales when I can, and if others are selling cheap, I buy. Unless you have a very large inventory of names, your sales fluctuations have more to do with chance than with whatever period it is.