Many traditional beliefs attach to various name days, especially involving the weather and the appropriate times to perform seasonal agricultural tasks such as planting some particular crop. For example there is a saying that "Jaakko (James) casts a cold rock into the water", meaning that on Jaakko's day, the 25th of July, the waters start getting colder.
It is customary to congratulate, send a card or give a small gift on someone's nameday.
The Swedish namedays differ in Finland and Sweden
Here is a list of the Finnish names and dates (in Finland)