None of this replaces treatment, and a forecast is not a diagnosis. What it can do is tell you which days are worth planning around.
Tree and grass pollen usually peak in the morning and again in the early evening, when air that rose during the day settles back down. If you can move a run or a commute out of those windows, that is often worth more than any other single change.
A storm can rupture pollen grains into fragments small enough to reach the lower airways, which is why some people have their worst attack during rain rather than in the sun before it.
Air the house at night or early morning when counts are lowest, and keep windows shut through the afternoon. In a car, recirculate rather than draw outside air in.
Pollen travels indoors on clothes and hair. Changing when you get home and washing your hair before bed keeps it off the pillow, which is where it does the most damage to sleep.
Seasons shift by three to four weeks between southern and northern Europe, and the dominant species changes entirely. A birch-allergic traveller has an easier May in Andalusia than at home in Sweden; someone allergic to olive has the opposite problem.
Most preventive antihistamines and nasal steroids work better started a week or two ahead of the season than once symptoms have set in. The calendar page shows roughly when to expect yours.