How would you explain ‘for once happy’ in the context?
On the way back from the Colonia Roma dress shop today, it started to rain like pitchforks, and we bought paper hats from the newspaper boys. When it rains, they give up on shouting about the New Bureaucratic Plan and fold it up into something useful. Then we lost the way home and Mother laughed, her hair stuck like little black ribbons to her face, for once happy. For no reason.
How would you explain ‘for once happy’ in the second to last sentence?