Availability Calendar

This is a simple project, I need an availability calendar I can post to a client's website that I can highlight certain days. Something like you would see on a rental site. Preferably a several year or perpetual calendar. I would like to have 3 or more months viewable at a time. I need this to be super simple to update preferably just a click to highlight the dates I need and something easy to load onto my client's site. Thanks Jill