Content Calendar or Editorial Calendar is something else that people begin to think about around the beginning of the year, but you can really start it whenever you want. It's really just a planning device that helps you plan out your content, so you don't have to feel overwhelmed and panicked. Most calendars are just a spreadsheet, with the dates in rows and content in columns.
Why do it?
You can plan your content around events that are important to your industry.
You can see where you'll need to find content to fill the gaps.
You will be ready and be able to publish on time.
What to include?
Start by listing all the holidays of the year. This way if all else fails you can relate your content to a holiday. You can also list events that are important to your industry. For example, if you have a food blog, you can list the SuperBowl to plan out snacks, or the Oscars and find food related to that year's nominees.
You should aim at posting at least once a week. Which really means only 52 blog posts, that can be written ahead of time and you can schedule them once their written. Technology is beautiful. Once you decide how many times you'll post, be sure to keep consistent, so your readers will know when to look for your information.