You can now save days to your library while working in a trip. Click the ellipsis at the top of the day you want to save.
You can edit the title for this day or "multi-event" and add a label. The label can be useful to organize items that are similar but it isn't necessary to add a label. Click "copy to library" to save the day to your library.
In the library search bar, you can type the label or the "multi-event" name.
Finally, click and drag the item to add it to the correct day or hit the "+" sign on the item. Once added, you can edit the title of the day.