This is a Calendar 29 icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,300 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.