The speaker of "A Birthday" is having a good day. Sure, most folks enjoy their birthdays, but we get the feeling that turning another year older is not literally what she's celebrating here. She's got something a bit more metaphorical in mind.
Now, we say "she," but there's really no clue to whether or not the speaker is a man or a woman in this poem. We're not really diving into someone's background. Instead, what we do hear about is the speaker's relationship with a certain special…someone.
This relationship has the speaker happier than kid in a candy store and more satisfied than a Tasmanian devil at an all-you-can-eat buffet. (Okay, so maybe we made that last one up.) The point here is that her life is peaceful and full: "My heart is like an apple-tree/ Whose boughs are bent with thickset fruit" (4-5). So, what's the cause of all these good feels?
Well, we learn through the poem's refrain—"my love is come to me" (8, 16)—that the speaker's experiencing a sense of closeness to a particular person. Given all the religious symbolism and imagery going on here (check out "Symbols" for the details), we'd say that it's a safe bet to assume that she's talking about her relationship with God. After all, in a religious sense, God is responsible for everyone's birthdays (see "What's Up With the Title?").
At the same, the speaker never actually mentions God in this poem. That's caused some critics to speculate that Rossetti was actually talking about a mortal man, some love interest or another that she had in her own personal life. That's definitely possible, and we're open to listening if you want to make that argument (we like to think we keep an open mind).
Still, the Christian symbolism seems pretty convincing if you ask us. That, coupled with Rossetti's well-documented Christian faith, which she practiced in her own lifetime, tell us that our speaker has found God—as is darn happy to have done so.