Question

Has given my heart
A change of mood
And saves some part
Of a day I had rued

Answer 1

Here Frost may have used the remaining day as a symbol for his remaining life(out of the sorrowful one).

With this Frost is trying to convey that the incident of snow falling from the tree has turned his sad and negativity filled mood to a better one. And has given him a kind of a hope and positivity to live through the remaining life happily.And this hope provided by that incident may not have directly but indirectly secured his remaining life which formerly he had ruined due to things like ill omens, superstitions and by the consideration of life as sorrowful and joyless.