New banner units

Finally...a question I can actually answer properly.

So, whenever a new banner comes out, the info on it will be displayed in the Notifications board in the top left corner of your home screen as well as the "More Info" button on the "Summon" page.

In the case of Legendary banners, there will be some blue text that should go along the lines of "Notes About This Summoning Event" on the aforementioned pages listed above. There, it will list the months for the next time that specific legendary hero will be available again for summoning. It should list the hero's name and then "late (insert month here)" next to it.

Hope this helps you out. Remember: all the info for every legendary banner can be found in "Notifications" as well. Just keep going back through each announcement until you find the one made for a past legendary banner and tap "More>" to get the information.

Edit: I forgot to also mention one other thing: For the most part, the units that come back for the banner are seasonals and some 5*s that have either been forgotten and haven't been introduced in a while or are showing up in the order of their initial banner appearance. The latter applies to the seasonal units that show up on these legendary banners, but there have been a few cases where this wasn't true.