This Calendar 1 icon is a responsive icon. This means that it's optimized for four resolutions, from tiny 16x16 until highly detailed 80x80. The style comes from the latest Microsoft Office, which makes it perfect for any Windows applications, in particular, those with ribbon toolbar control. Unlike other icon packs that have merely hundreds of icons, this monster icon pack contains 15,600 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.