Many businesses see their profit fluctuate with the seasons, especially during the holidays.  In fact, there are plenty of retailers dependent on the holiday season for the majority of their earnings.  Moreover, there are other companies that do best in warmer months - vacation destinations, for instance.  Therefore, it is only normal that companies see fluctuations in their website traffic depending on the time of year.

It is important to remember that even the best search engine optimization campaign is not immune to seasonal changes; after all, very few people are searching for Christmas gifts in August or for ski trips in July (unless they are in the southern hemisphere, that is).  Therefore, if your company is highly dependent on seasonal business, your website traffic will naturally drop off during the off months - and of course, it will increase during the prime time of the year.

Furthermore, seasonal website traffic fluctuations can occur for businesses that do not immediately seem like they would be affected by the season change, like camping equipment suppliers, boat manufacturers, or even things like hot chocolate that are simply more popular at certain times of the year.  Accordingly, it is important to take that into consideration when you look at your traffic statistics - and be sure not to panic when your site drops in the off-season, because it will certainly spring back up when it is more relevant again.