In reviewing a few announcements from a variety of sources including EuroControl, it looks as though ATC strikes will be quite popular in Europe during July.
Instead of putting all of them into one or 2 paragraphs, it is easier just to list them since there are several (times are in local time zones):
July 5: 11:00a to 3:00p
July 23: 10:00a to 6:00p
June 28: General strikes will have widespread outages.
Portugal (The following dates will have random 2 hour strikes held each day):
Keep in mind that these dates can come and go as negotiations go on between labor and management continue.