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 is the Calendar 13 icon for Windows 10
. Here we go. Optimized for 32x32 px, these are the simplest icons in the line style. Downloadable formats are PNG and SVG, among others. Icons8 icon pack is unique in providing a huge number of icons designed by the same team of four designers who create the new icons every working day in the same style and with the same quality. Same cool as the Bat Face, huh.