Traditionally, an election is called upon the party-in-power's election of a new leader (For example John Turner and Kim Campbell) so that the new leader has a mandate from the people to be prime minister. But, strictly following the rules, an election can be called by:
1) The acting prime minister (Trudeau) at any time
2) Waiting until the end of the term (October in Trudeau's case)
3) A vote of no confidence in the government (Not possible until at least end of March because Trudeau prorogued Parliament)
Trudeau has control over all three of these options (He can avoid the no confidence vote by extending prorogation all the way to October if he wants to). But Trudeau is under no obligation to communicate his plans.