For something like a hair cut, or standard dentist visit, maybe the day before. If they're nervous, we'll chat about it, watch a video, role play, etc. For my eldest, she had to have surgery, so we got some books, talked about the procedure and what would happen, who would be there, etc.. and we prepped her for a few weeks..
Usually the day before. I show her a video or whatever if she is unsure (eg fireworks)
It depends on the social maturity of the child I think. When my little one was two I wouldn’t prep but now he’s 3 nearly 4 and I do. Normally it involves ‘if you get your hair cut I’ll give you an ice cream’ 🤣 I prepped the three year old injections and he didn’t even cry but only a couple of days before and kept talking about it before it happened