Green’s Dictionary of Slang

day’s dawning n.

also day’s a-dawning
[rhy. sl.]

morning.

[UK]J. Franklyn Dict. of Rhy. Sl.
[UK]S.T. Kendall Up the Frog.
[UK]P. Wright Cockney Dialect and Sl. 98: Days a -dawnin’ ‘morning’.
[UK]R. Puxley Cockney Rabbit.