Though allergy season tends to be similar at the broadest level, allergy seasons are in reality highly local and specific, and Callergy is a tool that helps people to know if they need to be prepared if they find themselves in a new city. Callergy provides detailed charts that show the various allergens throughout the year in cities throughout the US.
Generally speaking, the spring is the worst time of the year for people who suffer allergies. That said, places with less rigidly defined seasons might have completely different allergy seasons, and Callergy allows people to give themselves some cursory preparation time.
On top of its charts for the cities it has listed, Callergy has more detailed written descriptions of the allergy conditions each month in a given city.