Tabor shows the deified humanity that the Dormition then delivers to one person. Disclosure of the principle over its first instance.
The Dormition is the first fruits of the general resurrection in a particular body. Trinity Sunday commemorates no event at all.
The birth of Christ against the death of his mother. Not close on any tradition's reckoning, whatever the Dormition fast implies about devotional intensity.
Neither is scripturally dated, but one is ancient and universal tradition and the other has a papal bull with a date on it.