Not everyone is affected by seasonal allergies, and they tend to affect everyone differently. One of the most common types is allergic rhinitis, also known as hay fever. Most of the time this allergy is caused by pollen in the area and occurs during the spring until the end of May. Other things other than pollen can also trigger hay fever throughout the year. While pollen affects people late in February until May, grass can trigger hay fever in other people from May until July. Finally, hay fever can also be triggered by ragweed and mold spores between July and November.