Any particular date when Postdoc decisions are announced in European universities?
Let me start with UK, as this country I know best. Unlike US, in the UK research grants are awarded by several large funding bodies and there is no particular date when all the decisions are simultaneously announced.  Hence, it is not possible to syncronise postdoc hiring process across the UK, and AFAIK, most professors do not see any need for it. Still, many Universities will advertise new positions for staff two times per year, with the anticipated starting date on 01 Oct (start of term) and 01 Apr (mid-term). There are two "waves" of recruitment, but they are not completely in sync.

In the rest of EU, customs differ but still I never came across anyone following AMS deadlines in the EU. It seems that people advertise as soon as they have funding confirmed, but different funding streams announce at different times.