According to the Patch Notes the downtime for the EU servers will be…
As with previous content updates, there will be server maintenance starting on April 15, from 5 p.m.–7 p.m. PDT for Oceania, East Asia, Southeast Asia, and Europe servers
That time equates to…
02:00 - 04:00 CEST on the morning of Wednesday, 16th April
However, on the BattleNet Launcher the downtime alert says…
The Launcher’s alert has the wrong day in the announcement.