I have no idea what this is about, but I can assure you that "the right time" somehow just never arrives. You create your own 'right moment' all by yourself. I guess what I'm saying is that conditions are never exactly comfortable, and it is your own courage and clear-seeing/thinking that create the ideal conditions for having Very Important Conversations.
Also - if you're worried about someone else's reaction - the longer you wait to have the conversation, the more likely it is that you'll have a build up of emotion or angst about it all, and the chances of garbling the message or coming across as angry or upset, increases.
In the meantime, no matter what the topic or how difficult it is - you can *always* remember to be extremely kind about it all. If you need to tell someone that you're bothered by something, or if you are about to possibly let someone down or let someone go, the best thing to do is to be extra caring about it all - being very clear, while also sort of stepping outside of yourself and imagining yourself as the recipient of said conversation.
This will make things go a lot more smoothly. You can give the other person the benefit of the doubt while still making your point.
Like I said - I have *no idea* what the nature of this conversation is, so I'm kind of guessing around here, but trying to offer up at least some 'pep-talk-ish' advice anyway.
Good luck with the conversation. You can do it! If it's that important, you aren't doing anything wrong, and the sooner you get this over with, the better