Four-day museum strike begins at Big Pit in Blaenavon
The first in a series of strikes over the Easter weekend begins on Friday.
Members of the Public and Commercial Services (PCS) union are taking action at sites across Wales in protest over plans to stop extra payments to weekend and bank holiday staff.
Industrial action starts at Big Pit in Blaenavon, Torfaen.
National Museum Wales said it faced a 4.7% cut in Welsh Government grant aid and could not continue payments in their current form.
Wales secretary of the PCS, Shavanah Taj, said: "The dispute has dragged on for two years and morale is at rock bottom.
"We understand that the museum's budget has been cut but don't accept it should be the front of house staff, the poorest paid in the organisation, who should make the greatest sacrifice, by giving up their weekend pay."
On Tuesday, National Museum Wales said the union had backtracked on an agreement to ballot members over a proposed resolution.
A museum spokesman added: "Resolving this dispute is now a matter of urgency, due to the pressures of a reduced budget. Therefore, it has no choice but to proceed to individual consultation."
- Big Pit National Coal Museum will open at 13:00 GMT but with no underground tours on Friday or Monday
- The National Wool Museum, in Llandysul, Carmarthenshire, will be closed on Saturday
- The National Waterfront Museum's galleries, in Swansea, will be shut on Saturday
- Some historic buildings at St Fagan's National History Museum, in Cardiff, will be closed on Sunday
- The National Slate Museum, in Llanberis, Gwynedd, will be shut on Sunday
- Treasures: Adventures in Archaeology exhibit and galleries will be closed at Cardiff's National Museum Wales on Monday