A day break separates one shooting day from the next on your stripboard. To add one, click the calendar plus (+) button at the top of the Stripboard panel; it's placed after your selected strip (or at the bottom), and you can drag it to reposition. If your day-break design includes a duration, it totals automatically from the strips within that day.
For everything about day breaks (start/end-of-day banners, units and multi-unit day breaks, text banners, and controlling colors, borders, and fonts), see Banners & day breaks: customizing your stripboard.