This is the number three in the middle of a square. There is a line drawn horizontally across the top of the square. There are two little tabs attached to the top of the box.
This Calendar 3 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 5,700 icons, all in the same style and quality.