Milestones let you mark important events directly on your chart, so you can quickly understand what might have caused changes in your traffic.These are especially useful for things like real-world events, website changes, campaigns, or anything that can’t directly attribute in your data — like running a physical advertising campaign (billboard ads, etc.), launching a product, or getting featured in a YouTube video. With Milestones, you can now add context to your analytics that only you could know.
From your dashboard, click Settings in the top right
Click on your site name
Clcik the Milestones tab
Click Add your first milestone (or Add new if you’ve already created one)
Enter a name and date for your milestone
Click Create
Your milestone will now appear as a small marker on the date you selected at the bottom your site’s chart. Hover over the marker and a tooltip will appear showing your milestone.