Final answer:
To describe a physical state using the verb estar + adjective in Spanish.
Step-by-step explanation:
When describing physical and mental states in Spanish, we use the verb 'estar' with adjectives and the verb 'tener' with nouns. In the given sentence, 'Es mi cumpleaños. Vamos a hacer una fiesta con todos mis amigos,' to describe the physical state of being happy, we would use the first person singular form of the verb estar, which is 'estoy'.
Learn more about Describing states using estar + adjective in Spanish