Your celebrations are your personal video files which are playing on the TV-screens when you have reported an activity that should be celebrated. They accompany your moments of glory, so make them truly yours by uploading your own custom music/video. 

First, click on Profile settings from the dropdown menu in the top right corner:

Next, click on Celebrations

Once on you celebration management page, click on the plus sign to add a link:

From here, you can copy+paste the link to a YouTube video. Paste the link in "YouTube link"- section.
Name the video whatever you like in the "Video title" section.

You can set a desired start time in the "Start time (in seconds)" section, so that your celebration will start at a specific part of the video.

Lastly, you have the opportunity to mute/unmute your video.

Click on "Save" and you are all set.

Note: You can set different videos to different kind of activities.