This functionality was deferred from the initial On this Day release. It should be ready for dev with no further design assets required.
Transition was split off from T175995

Design details:

• The top most visible 'years past' tag is shown as "active" by a different background color
• Upon scrolling down, the next tag in the list "bumps" the active tag off screen and replaces it as the top active tag.
• Event content 'fades' as it scrolls behind the active tag:
  

Additional resources

• Open full screen to see animated GIF:
  
• Invision prototype: https://wikimedia.invisionapp.com/share/SKEE7H9CP
• Redline mocks are shown tagged with "On this day" on Zeplin board https://zpl.io/GaRQKKV