The calendar displays the current season currently affecting the world. Note that this does not take into account blacklisted dimensions where seasons do no occur.
Each season graphic is shown at the left as an animation, a butterfly for spring, a sun for summer, a yellowing leaf for autumn, and a snowflake for winter.
There are also seasons for the Nether. Blaze powder for sparks (spring), a flame for bonfire, a blaze rod for embers, and a lump of obsidian for freeze ("it's a cold day in hell" even if its still 280 degrees hot).