This is a High Priority 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 12,600 icons, all in the same style and quality.
We've received the idea on November 26, 2012 and created the icons on February 24, 2013
A set of icons for viewing a calendar/schedule, appointment reminders, recurring appointment icon, "show as" icon, categorize icon, along with priority icons (high, low and private). See this screenshot for concepts: