This is a Calendar 1 Filled 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 12,800 icons, all in the same style and quality.
It's a square with two ring-like objects attached to the top, to indicate that this is a calendar that would flip to change the date each day. Inside the square is the number 1, to signify the 1st day of the month.