Using a keyword research tool like Google Trends or KeywordDiscovery.com, one can see the seasonality in search term popularity. For example, as one might expect, gourmet chocolate related searches peak at the holidays, as well as around Valentine's Day and Easter.
The ebbing and flowing of searcher activity is also evidenced in your website stats. Search referred traffic will drop in the low season, unsurprisingly. But would you have expected to see the relative share of each search engine to change depending on the time of year?