You can create a post and save it for later by creating a draft. To do that:
Open your calendar section from the menu bar on the left.
In the upper right corner, click on the + New Post button.
The creating post box will be displayed.
Create a post by writing the content and adjusting it to each chosen account.
Finally, at the bottom of the creating post box, you will see the Add draft button.
Click on it to save the post as a draft.