Historically /technically cocktail hour starts after 5pm and finishes by 7.30pm - 8.00pm. It is supposed to be a transition social,gathering. That is you are either going to cocktail do after work on the way home, or from home to the opera or dinner or such. So if you come from work you wear your work outfit, which was always a suit, or if going to opera you would wear your opera outfit which should be a Dinner Suit. Both outfits are acceptable and in fact correct. It is a transition Occassion.
However times have changed, dudes are likely to be wearing ill fitting tight jeans or worse to work on Friday , usual cocktail day, with a hoodie top and band t shirt. Clearly not cocktail wear even if transitioning.
Depends on the season, but I’d say toward summer a smart linen suit, or if your tendencies are a bit retro rock n Roll, then a white or ecru dinner jacket with the black trousers would be hip, or winter say a spiffy velvet jacket, deep burgundy or navy or even deep green, with tailored trousers. Naturally anything associated with opera, even tangentially, says dinner suit will always fit in.
But hey, knock yourself out. If it’s anything like here or USA then most people will look like they were working in the garden an hour ago and said “Fuck it, let’s go to the cocktail party fwiffo mentioned. There’s no time to change clothes or have a shower. Jump in the car”
