A QUIET PROTEST
Agendus is the gerundive form of ago, which has a lot of meanings. Here we should think of it as to spend in the sense of spending time. The gerundive gives a verb the sense of should, or must. Read more about it here. Natalis qui agendus est: the birthday must be spent.
Rure is in the country. It comes here with the adjective molesto, which you might guess means annoying. Sine is a preposition which takes the ablative. It means without, and Cerintho is the ablative form of the name Cerinthus: the boyfriend. That leaves tristis, sad, another adjective modifying natalis.
Translating a gerundive literally (“which is to be spent”) is a useful step, but it usually makes for a smoother translation when we ask who is involved. In this case it’s the poet herself, so let’s render it directly as I must spend.
Invisus natalis adest, qui rure molesto
et sine Cerintho tristis agendus erit.
My stupid bloody birthday is here, which I have to spend in misery in the stupid bloody country without Cerinthus.
I’m making some pretty free choices here. Once you see how the Latin works, you should play freely with your own translation. The vocabulary will help. This poem, more than any piece of Latin I’ve seen, reads to me with the immediacy of a diary entry. The slangy translation is no slight on the writer, who is just as technically adept as the other poets we’ve looked at.