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I agree with the prior comment. However, I'd like to add something.

It can also mean that we have been granted (in this case by God or nature) the ability to be at peace or experience happiness even in the bleakest of times. The roses can symbolize anything which is positive. Our minds are powerful instruments.

Since roses aren't blooming in December, you can still enjoy them if you have the memory.

Looking long term, if you think about December as being the end of life, then the roses could be metaphores for many of the joys of youth.

December is the last month of the year and if we can't remember anything remarkable that happened during the year then there are no roses at the end of life, nothing to look back on. Nothing at all.

you can't grow roses in December, but you can remember the roses of the summer, then you'll have roses in December.

means that when we are old, we can look back & enjoy memories of our happy young /past days & all former happinesses that we can bring to mind.