The circular calendar visualizes the possible hours of sunshine at latitude 50 degrees north of the equator for 2020 with the yellow area inside. Two additional thin rings (solid and dashed) mark the sunshine hours of the 40th and 30th degrees of latitude north of the equator.
By a concentric hourly scale, the possible hours of sunshine for the days in 2020 can be read. All around, the calendar displays all months and days for 2020, but also the calendar weeks, moon phases, as well as summer, winter, spring, and autumn solstices are displayed. It additionally offers enough space for daily notes.
FF DIN was chosen because of the very good readability of the numbers. Even when viewed from further away, numbers are still well recognizable. Not for nothing was DIN formerly used with German license plates. I love this font :)