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.
This Calendar 13 icon is made in flat color style
. This format was developed by Icons8 designer team and became immensely popular for use in web and mobile applications. Feel free to download this icon in PNG format for free, or unlock other formats for $5. Unlike other icon packs that have merely hundreds of icons, this monster icon pack contains 6,000 icons, all in the same style and quality.