Facebook is doing away with the Messenger Day feature within its messaging app and now synchronizes it with the regular stories feature on the Facebook app. This means the Facebook now has the same stories across both apps.
To make it more clearer to you, henceforth, the Stories you see in the main Facebook app will now be the same ones that you see in the Messenger app. If you upload them in one app, they are visible in the other app. If you open them in one app, they are marked as viewed in the other app. They now get synchronized and works together as one.
This measure is to make it more user friendly and eliminate the extra burden of uploading pictures twice (on Facebook and messenger) in the name of day stories images.
Meanwhile, Facebook is as well reportedly working on enabling cross-posting Instagram Stories to Facebook but the reverse is not yet planned. Kindly note that Facebook owns.