What should I wear to my birthday dinner party?

My mixed group of guy friends and girl friends are going to celebrate my thirty sixth birthday dinner party next month. There are five of us. It’s spring here. What should I wear in the evening? It’ll be sort of chilly at night, I imagine. We’ll be at a casual diner, that makes good, but not great food, at a reasonable price. I’m not interested in either of my guy friends romantically, so I may not need to dress up?

What should I wear to my birthday dinner party?
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