It's your birthday.
And mother's crying. Just like every year.
Yes, she is allowed to mourn the past, but it is still your birthday and you're allowed to be angry about being overlooked.
Plus, grandmother had promised to come and she didn't. Perhaps she was eaten by a wolf on her trek through the woods... or perhaps grandmother, too, has forgotten about you.
Either way, you have an excuse to get out of the house. Mother can stay here and grieve. You'll make a trip into the woods to check on grandmother. You'd rather spend your birthday with her anyhow.
Grandmother's always seemed to understand you better than anyone...