How it is studied
A calendar summarises when each taxon usually occurs in an observation series. Interpretation requires the station, study years and rule defining a season. This website's regional table is a predefined teaching guide, not an analysis of a local time series.
An example to interpret it
Example: a calendar places a season between March and May. That helps planning but does not give today's concentration. For that question, consult recent observations or a clearly labelled forecast.
What to check
An uncoloured cell does not guarantee pollen-free air. Altitude, long-distance transport or an unusual year can shift or extend a season.