"It is said that God gives us certain moments so that we may have roses in December."

I'm more familiar with this version:
'God gave us memory so that we may have roses in December'
(James Barrie)

It means that because we have memories and can recall things, in the depths of cold, dark winter, (or sadness or grief) we can comfort and warm ourselves with the memory of something beautiful and/or calming or comforting - like roses (or a happier time).

I guess for people in the southern hemisphere - you'd have to change the month so it would make sense though.