This Kalendarz 13 icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 4,300 icons, all in the same style and quality.
The image is of a notepad or calendar with the number thirteen. There are two little tabs at the top of a square which would be the binds for the notepad or calendar. The number 13 is just centered in the bigger part of the square sheet.