Se ho capito bene la domanda, con [:start, :end]
essendo il tuo intervallo di date di interesse, stai cercando:
select *
from event
where -- event started earlier, ends later
start <= :start and :start <= end
or -- event starts during [:start, :end]
:start <= start and start <= :end
or -- event ends during [:start, :end]
:start <= end and end <= :end;
Se stai cercando un particolare :day
, usa :day
come :start
e :day + 1 day
come :end
.