The best way to add a calendar to Google sheets is to make a template. This just takes about an hour to make the way you need. But once it is done, you have it forever. Things to have in the calendar are as follows; the social media platform name, the date, and the content you plan on publishing. If you need something fast and already made, OWLLytics already has one built for Google Sheets for 2021. You just need to make a copy to your drive and you are off to the races.!!