The Dormition is the first fruits of the general resurrection in a particular body. Trinity Sunday commemorates no event at all.
A feast of the doctrine that organises every other feast beats a feast of the instrument, though the Cross has the better paradox.
The Ascension is an event with consequences for what human nature now is. Trinity Sunday is a rubric for thinking about the rest.
An event in the economy of salvation over a commemoration of doctrine, even a foundational one.
The event that grounds the doctrine outranks the day set aside to state the doctrine.
Even a feast of pure doctrine outranks a feast of a devotional practice, since the doctrine is the reason the practice exists.