这是一个现在进行时的特殊疑问句,句中其实是要表达一般将来时的含义。
来去含义类的动词,如go, come, leave, arrive等经常用进行时表示将来时。所以这句话的意思是:他们打算什么时候去博物馆?
回答为:They are going to the museum + 将来时间。比如:
They are going to the museum this afternoon/ this evening/ tomorrow/ the day after tomorrow/ next Monday.