Seasonal keywords will play a hugely important part of the keyword research process for SEO, PPC and Content Marketing specialists. When mentioning the word 'Seasonal' traditional images of Turkeys, Christmas trees, Menorahs, Halloween costumes or Valentine's day chocolates tend to appear.
Many Western cultures associate traditional calendar events with holiday periods around the November and December months so it's unsurprising that this period provides a huge number of Western retailers and ecommerce businesses with a large chunk of their annual revenue. Black Friday was named as such because it's the day many retailers go from being 'in the red' to 'in the black'.
What all this points to is that getting the seasonality of your keyword strategy right should be a key focus to ensure you maximize the potential of each keyword's seasonal strong point. Furthermore, it's important to remember that almost all keywords, not only those associated with a holiday, will have a distinct seasonality of their own.