First off, congratulations to Meryl Streep. Based on this picture, it would appear that she and her date for the evening were voted as prom king and queen! We assume that's the case anyway, because otherwise we're not clear on why they're taking a walk through the gardens with an audience, and she's wearing a tiara. If we're right about them winning, it's the ultimate vindication of Streep's bold choice of dress.
A few of the style choices on this list were products of their era, and this is another one. It would have been the 1960s when Streep went to prom, and dresses like this one were all the rage. We shouldn't hold that against her; we imagine it was quite a common look on the night. She looks like a hippy and a flower child; perhaps she was in training for the summer of love, which was right around the corner!