The new opt-in feature will help you reminisce past road trips, family gatherings, and more, by displaying a collage featuring people and places from that day. The more photos are uploaded to your library, the more useful the new card will be at helping you rediscover the good old days. And as a reminder, those snaps of years past will display in Assistant, and is completely private unless you share elsewhere.
The feature is currently rolling out to Google Photos on Android, iOS and the web.
Source: +Google Photos - Google+ post