Several things in your student life may impact when it is best for you to start an internship.
Sometimes it is best to do it just before you have to write your final project, for example, because you need your study plan to include the courses you are missing to be allowed to do an internship.
It may also be that it makes better sense to save some of your elective ECTS points until you have completed the internship so that the internship can inspire you to choose your final elective courses.
An engineering internship can often give you a unique insight into the working life you will encounter when you graduate. The internship can help you learn more about yourself, your professionalism and what types of tasks you would like to work on after graduation.