Postlets has no way of scheduling an open house through their system, however if you search the topic in their help menu they suggest entering the open house date in the description.
Just keep in mind that once the open house passes it will look silly if the open house has passed.
Postlets makes it easy to post your postlet onto Craigslist.org. Select “Links” under the drop down menu on the “My Postlets” page.
It helps to make up a catchy name for the property on the Craigslist title- don’t just say the address. Maybe say, Beautiful Home with a View, or something like that.