Answer: the passage of time
Step-by-step explanation:
The whole idea of this poem by Robert Frost titled, ''Nothing Gold Can Stay'' is that as time goes by, beautiful things will eventually wither, decay and die so it is best to enjoy them whilst one still can.
The alliteration: "So dawn goes down to day", represent that passage of time because as the hours in a day go by, dawn eventually becomes day signifying that time has gone by.