Originally Posted by wiki

If a flight is cancelled, passengers are automatically entitled to their choice of (a.) re-routing to the same destination at the earliest opportunity (under comparable conditions); (b.) later rerouting, at the passenger's convenience, to the same destination under comparable conditions (subject to seat availability); or (c.) a refund of the ticket as well as a return flight to the point of first departure, when relevant. Any ticket refund is the price paid for the flight(s) not used, plus the cost of flights already flown in cases where the cancellation has made those flights of no purpose. Where applicable, passengers are also entitled to refreshments, communication and accommodation as described below. Where re-routing is to another airport serving the same destination, the airline must pay for onward transport to the original airport or to a close-by destination agreed with the passenger. These choices, and the entitlement to refreshments, etc., apply to all cancellations, regardless of whether the circumstances are extraordinary or not.

The airline is also required to pay cash compensation as described below, unless one of the following conditions applies:
the airline notifies the passengers at least two weeks prior to departure
the airline notifies the passengers between one and two weeks prior to departure, and re-routes passengers so that they can: depart no more than two hours earlier than scheduled, and
arrive no more than four hours later than scheduled

the airline notifies the passengers less than one week prior to departure, and re-routes passengers so that they can: depart no more than one hour earlier than scheduled, and
arrive no more than two hours later than scheduled

the cancellation was caused by extraordinary circumstances that could not have been avoided by any reasonable measure.