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What type of suit should a man wear to a summer "Garden party" wedding reception in August? - garden party dresses

This reception is an elegant residence in the United States, such as, for example, instead of after the wedding, held in England (Henley on Thames) this spring.

It is welcomed as a "Garden Party", but the dress to be "elegantly formal." Reception starts at 5:00 clock in the evening.

Is it right for my husband to wear formal black suit and a silk shirt or color
it would be appropriate, perhaps, a bright dress with this event?

A rule of thumb or advice in this regard highly appreciated.

Thanks