Further to this thread- https://community.ricksteves.com/travel-forum/scotland/st-kilda-from-barra-
Booking is now open up until early August on Hebridean Sea Tours-
Mingulay trips run on most Mondays and Wednesdays, also some Fridays from 7 May.
The St Kilda trip is being run on 2, 9 and 30 June also 7 and 28 July and 4 August.
It may well be that 4 August is the last of the season, given that the Scottish schools go back a week after that. They may well feel that their primary customer base is the domestic market, not the international one. Also the puffins have gone by early August. Although there is more to Mingulay than puffins and other sea birds that will be the big draw.
They either run Mingulay or St Kilda on any given day, so substitution for the other destination isn't an option.
So the answer to the original question of when do they open bookings seems to be- early in the New Year.
There are also short non landing trips around Kisimul Castle (in the bay at Castlebay)- the Historic Scotland website tells you very little but last Autumn they were advised to be running on Thursday to Saturday several times a day- just turn up at Castlebay marina at a simple unassuming sign, pay and board.
There is also an exhibition about Kisimul Castle and it's history in Barra Community Hall. As far as I know that is a permanent exhibition.
From early 2026 these should become landing trips, when the Castle opens to the public.