Kalendarz 1 and other 80,800 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
It's a square with two ring-like objects attached to the top, to indicate that this is a calendar that would flip to change the date each day. Inside the square is the number 1, to signify the 1st day of the month.