After working on something similar for a client several years back (see post here) and getting tons of requests, I finally released the Blog Post Calendar Widget plugin. The monthly grid-view calendar can be added as a widget and highlights both archived and/or future posts.
Want to display upcoming events in your sidebar? With the ability to show posts by post type, category, specific taxonomy and/or term, you can opt to display future posts assigned to your event category or event post type.
You can download the Blog Post Calendar Widget for WordPress here.