Hola Matthew! The second example would be: "¿En qué fecha vas a viajar a México? Hope this helps! :)

Estoy de acuerdo con Gabriel. Si bien "¿Para qué fecha vas a viajar a México?" no está mal (por lo menos en Argentina podemos usar esta frase), "¿En qué fecha vas a viajar a México?" suena más natural. Saludos!

PARA has a limited number of meanings in Spanish. These include: a destination, a recipient, a purpose or goal, expressing "in order to", a specific future time for which something is intended, a deadline, a use or function, a comparison & contrast, and an opinion. Para is also used in "estar para" to express the idea of an action soon to be completed. In your example "para qué fecha" refers to a specific future time for which you want the tickets. A native speaker once explained to me that "por" looks back in time to the cause of something, whereas "para" looks forward in time to the effect of something.