You can move a snippet to a different day on iOS and Android, but how you do it depends on the kind of project. Freestyle projects have a Change Date option; Journal projects don't, so you need to delete the snippet from its day and add it again to the right one.
In a Freestyle project
Open the snippet in your Freestyle project.
Tap the three dots on the snippet.
Choose Change Date, then pick the new date and confirm.
On iPhone, pressing and holding the snippet in the grid opens the same menu.
The snippet moves to the new date right away. If you don't see Change Date, the snippet is in a Freestyle Collab project: collab snippets keep the date their photo or video was taken, and you can only rearrange them.
In a Journal project
Open the day the snippet is on and remove the snippet. Your original photo or video stays in your library.
Back in the calendar, tap the day you want it on first.
Add the snippet again from your library.
Tapping the day first is what matters. 1SE suggests photos and videos by the date they were taken, so a video from just after midnight can turn up under the previous day; choosing the day yourself puts it where you want it.
Freestyle projects need a Pro subscription. One Journal project is free, and here's how the project types compare.