Shotwell defines Events as 'A group of photos that were taken at approximately the same time.' This is a grey area, and things like batches of scanned images, multiple photo shoots with multiple imports the same day, or sets of photos taken around midnight, can leave you with an event structure that you might not prefer.
If your goal is to have Shotwell sort photos strictly by individual dates, then this is a long-standing bug report/feature request (#2747). Disabling events entirely and using date only is another closely related request (#2665).
Questions that are bug reports (and feature requests are a subset of bug reports) are off-topic on Ask Ubuntu so your question may be closed.
If you want to manually sort the images in question, creating events and moving groups of images in to each event, Shotwell's help page describes how. I'll summarize here but it's best to read that entire page.
- To rename an event select Events -> Rename Event or double-click it and type.
- To move photos between events, drag any photo to the sidebar and drop it on an event.
- To create a new event select Events -> New Event
If you find that any of the photos have the wrong date/time that's easy to adjust too.