This Calendar 29 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 4,400 icons, all in the same style and quality.
It's a square with 2 ring-like objects at the top, to indicate that it's a calendar that you would flip to change the date each day. There is the number 29 inside the square, to signify that it is the 29th day of the month.