Once the 2016-17 season is over - after the three remaining finals - the RFEF can design their calendar for next season, bearing in mind the Russia 2018 World Cup.
FIFA have decided June 14 is the fixed date for the World Cup start. That means the Champions League and Europa League finals will have to take place earlier than this year, on 26 and 16th May respectively. Which in turn means the Copa del Rey final and La Liga finales also must take place earlier.
It seems likely that La Liga will start on 19-20 August. In that case the Spanish Super Cup might be on 12-13 August and the return on 14-15, with the European Super Cup on August 8.
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The fixture congestion towards the end of the season might reward the sides with the deepest squads.