The summer school vacations in Germany start end of June / begin of July 2026, some later - depending on state. So, normally the people who are not depending on school vacations windows travel before and after these windows. Therefore the whole German North and Baltic Sea shores have top season and high demand. normally from begin of June until end of September - with annually growing window.
June 21 has the longest daytime, so a lot of usable time, also in the evening when on sunny days rest light here in the North does not disappear before 10.30 p. m.. These days play a role in the North, starting in Germany and more up to Scandinavia. In Sweden the second most important fest after Christmas is Midsummer.
Without knowing it I guess that Helgoland accommodations save capacities for spontaneous guests which pay a lot of money for the stay. I recommend to book a flex tariff which you can cancel to a certain date if better opportunity occurs.
As likely 80-90% of Helgoland visitors are German guests it might make more sense to lookup capacities by a German hotel engine such as hrs.de.
The ferry costs you can just look up.
Btw: Helgoland is the place where Hoffmann von Fallersleben had written the text of Weimar Republic and today's German anthem, known as "Das Lied der Deutschen" (link to a little story about it in German). Only the third verse is used today (link to text and music). At this time Helgoland was British property, the melody of orientation was Haydn's Austrian emperor anthem and in 1841 the Germany as we know it today was not even existing.
So, have a good journey.