This Calendar 1 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 5,100 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.