If you went on a continuous multiple day length sailing trip, for example a weeklong sailing vacation or such, you can easily make this entry into your logbook. Just click the checkbox "[ ] This is a multi-day trip".
For nights, enter the number of nights you actually sailed through the night - not at dock or at anchor. If it was a weeklong trip and you anchored or moored each night just enter "0" for the number of nights.
A multi-day entry is for a continuous sailing trip. It is NOT a fast way to enter say 30 days of sailing in one year.
The Multi-days function to the logbook was introduced in late 2018. So if you made multiple single entries prior to that for a, say, weeklong trip, then we do advise that you delete those older single entry days and make a new multi-day entry to that week (or however long).
Authentication works the same, your crewmates will be sent an email asking to authenticate the entire trip.
TrackLink and SailTies Apps integrate well with Multi-day entries as well. When you make a multiday entry, it will integrate any TrackLink entries into the entry and apply appropriate authentication to the entry.
A big advantage of multi-day entries is that it shows in your NauticEd sailing resume as a multi-day trip. This gives your sailing resume more power when wanting to charter a yacht for yourself.