The thing that puzzles me is that the house next door isn't decorated, and if it was a national event like a coronation/celebration surely the whole street would have joined in.
But would they go to all that effort for a wedding?
With the bouquets, however, I think I'd put my money on it being a wedding.
Perhaps the picture was taken during the decorating, before they changed into their glad rags to go to the wedding?
Or perhaps your gt grand's brothers' wedding was a long awaited event!
