Adding to the disruptions, unions at Brussels Airlines plan industrial action around the same period.
The ACV and BBTK unions said they were forced into action because Ryanair was not respecting Belgian labour law covering such issues as the minimum wage or pay of cabin staff for certain pre- and post-flight work.
They said Ryanair operated 17 planes out of Charleroi airport and two out of Brussels.
Unions representing some Ryanair cabin crew in Italy, Portugal, France and Spain have all announced strike plans in recent days.
A Ryanair spokesperson said on Friday that the unions should return to the negotiating table to deliver improvements instead of disrupting Belgian customers’ travel plans.
Ryanair Chief Executive Michael O’Leary had said on Tuesday that union accusations were “complete rubbish”.
“We fully comply with Belgian law, we have labour agreements here with both the pilot unions and the cabin crew unions,” he told a news conference, adding that during a strike in April fewer than 40% of Ryanair’s Belgian flights were affected.
More than 60% of flights to and from Belgium operate on aircraft that are not based in the country.
Separately, unions said on Thursday that pilots and cabin crew of Brussels Airlines, a unit of Lufthansa, would go on strike on June 23-25.