E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
White Moments of Life (The).
The red-letter days or happy moments of life. The Romans used to mark unlucky days, in their calendars, with black chalk, and lucky ones with white chalk; hence Notare diem lactea gemma or alba means to mark a day as a lucky one.
These, my young friend, these are the white moments of ones life.Sir W. Scott: The Antiquary, chap. iii.