We all are aware of Gmail adding new features now and then regardless of what the actual users want (the pain caused by the forced new features like Gmail Compose made me say that). But some of such additions by them result handy to the Gmail users and Pencil it in can be one of them.

What the heck is “Pencil it in”?

“Pencil it in” is a new feature in Gmail that lets you add calender events to Google Calender just by clicking the dates and time in your email messages. It is active by default in your email, you can give it a test by clicking on date or time in your emails.

Pencil it in - Gmail feature

If you are unable to use this feature on your Gmail, you may need to wait for some days; as Gmail has not applied it globally.