Birthdays are a good day for self-reflection. You have lived through another year and there have been great memories made during that time. And perhaps there will be a few thoughts about what you want to accomplish this year. But what does all this have to do with writing?
Everything. This is where you get the best ideas for your writing over the next year. Also if you wish to write down in a journal all the events of the previous year so that you remember them should you ever want to write a memoir, this is a good time.
But self-reflection is also good for poetry. I am a novelist with the occasional short story idea, but on very rare occasions a poem with come out. Usually in the form of a limerick, because they are the greatest form of poetry. There will never be a book of my poetry published, but poetry does help focus the self-reflection into something meaningful and useful.