Thanks for contacting Lingvist!
Although both words refer to a relatively close future, they are a bit different in meaning because they take separate time reference points:
próximo refers to the future from actions that start at the very moment of speaking. For example, if you say El próximo jueves me voy de vacaciones on a Tuesday, it means the action will take place not on Thursday that same week (two days after), but on Thursday the following week.
siguiente takes a previously-mentioned time element as reference point. For instance, the sentence Al día siguiente no recordaba nada de la fiesta, refers to an action that started before the moment of speaking (la fiesta).
A good way to remember the difference between these two adjectives is to create contextual examples:
– ¿Cuándo te vas de vacaciones?
– La próxima semana no, la siguiente (that is, in two weeks' time).
Thank you very much for your input!
However, we have to point out that rule is not always true regarding siguiente. Take the following examples:
- Tomaremos el siguiente autobús, ya que hemos perdido el anterior --> "We'll take the next bus as we have missed the previous one."
- (when queuing up) ¿Quién es el siguiente? --> "Who's next?"
Our advice is that you bear in mind the specific time references that each adjective takes to be able to differentiate them clearly ;-)