How do you say have a in Spanish?

"Have a" in Spanish depends on what you're saying "have a" before. Here are some common examples:

Have a good day: "Tener un buen día"

Have a nice trip: "Tener un buen viaje"

Have a happy birthday: "Tener un feliz cumpleaños"

Have a good time: "Pasar un buen rato" (or "Tener un buen momento")

Have a nice weekend: "Tener un buen fin de semana"

Have a good night: "Tener una buena noche"

You can also use "Ten" which means "have".

For example:

Ten un buen día (Have a good day)

Just be sure to adjust the wording based on what you're saying!