Risen Kingdoms is a turn based 4x styled game with strategic rpg battles and a persistent hero system.
Using the calendar menu is completely optional but can help when planning attacks on your enemies. The number of turns a day or night can last will very, meaning if your planning an attack involving units that are stronger during the night and weaker during the day, your going to want to move into enemy territory on a longer night rather than a longer day or shorter night.
Each month can also very in the number of days they possess, so if a month offers a particularly powerful buff that month may have slightly less days in it than usual.
Events can also be tracked here although you will not know what they hold until the day arrives. Events can very from a chest spawning on the map to a Spider Queen appearing in the scenario, these events can greatly impact the game as the scenario progresses on.