Step-2 :- Now click on Edit on that post.
Step-3 :- Click on Published on option on right side bar.
Step-4 :- Select the option Set date and time
Step -5 :- Choose future date from current date.
Step-6 :- Click on "Done" button.
Now your sticky post is ready this post will display on top until you reach the defined date.
Another method to create sticky post is using Gadget but according to me this is simple and better way to create sticky post.