Dagaz is the letter D

Anglo-Saxon Rune Poem (from ragweedforge.com)

Day, the glorious light of the Creator, is sent by the Lord;
it is beloved of men, a source of hope and happiness to rich and poor,and of service to all

Dagaz means day, as the poem indicates. It most strongly refers to the light of day, but it also encompasses an entire 24 hour period of both light and dark.
The esoteric meaning of Dagaz is related to time and to change, but also to what the return of day means, awakening both physical and spiritual