When to start contacting possible PhD supervisors (Europe)?
Can't give you a generic answer, but here is my particular situation: I am an assistant professor in the Netherlands. Contacting me to pursue a PhD is futile, since I have no funding at the moment to host PhDs. If such funding would become available (which can happen at any time; there is no rhythm to this), positions will appear on my university's vancancies page. You are most welcome to apply to any such positions, but contacting me beforehand is not necessary.
In all of the Netherlands, PhD students are viewed as university employees. Therefore, a PhD vacancy is not a matter of student admission, but a matter of employee hiring. As a consequence, the human resources department will manage the bulk of the procedure; although professors definitely get to choose people that would be a good topical match, the hiring process is much more centralized than in some other countries. In Germany, situations vary: at some universities and in some groups, PhD positions are 100% paid as jobs, while at other universities and in other groups, PhD positions are 50% employee, 50% student. I cannot vouch for the situation in Belgium and/or France.