We applications and websites get measurable traffic, recorded in metrics such as pageviews, unique visitors, and visits. All of this activity translates to hits to a webserver, and work for a database to retrieve information for those pages.
During one month your application might get 150,000 visits, then during one week where a large ad campaign hits, or some marketing feature goes viral, you suddenly get 500,000 visits in one week! This is a “good problem to have” on the business side, but poses great challenges to an infrastructure as it represents a 7x increase. What’s more if you do your capacity planning around that peak, you’ll have in 600% of your computing power and servers sitting around idle most of the year (assuming that’s just a blip).
Therein lies the challenge of seasonal traffic variations. Capacity planning attempts to watch for trends in traffic, and growth over time of your user base. But large spikes like the one described above can often be difficult to predict. The whim of the masses.