Hi Jessica, a little late to the party here. I recently built to help with promotions. It's not a promo calendar app, but can help you schedule your Shopify theme for your promotions. I use it for various Shopify stores that I manage to help swap out announcement banners and particular sections of my theme. If you're interested in learning more, you can read about how it works on my blog.
I'm also working on a larger app for running promotions and have considered adding a Promotional Calendar. I'd love to get your thoughts on what would make a good promotional calendar app. Do you have some time to chat on or offline? Feel free to email me at alex@ newclick.io (make sure to remove the space).