Historically, the Cannes Film Festival has always run from Wednesday to Sunday, starting in mid-May. Although this format has been in place for several decades, it's had the effect of focussing attention on the first weekend, and placing the last weekend (and the festival's awards ceremony) at a point where a large number of people have already gone home.
From 2018 (and presumably beyond), the Festival de Cannes will run one day earlier, starting on a Tuesday and finishing on the following Saturday. This makes the dates of the 2018 festival 8 - 19 May.
According to the festival, the intention is "... to rebalance the two weeks of the event and to bring new energy to the proceedings." It also means that the awards ceremony will now take place on the last Saturday night.