How can I restore favorite photos in Calendar app on iPhone Home Screen?

Re: "os18 was getting some favorite photos in a widget on my home screen, then it stopped.

How do I get them back?"

See whether Apple support's online article (link below) can help:

How to add and edit widgets on your iPhone - Apple Support

Eg: says:

"How to add widgets to your Home Screen

1. From the Home Screen, touch and hold a widget or an empty area until the apps jiggle.
2. Tap Edit in the upper-left corner.
3. Tap Add Widget.
4. Select a widget, choose a widget size, then tap Add Widget.
5. Tap Done."