We picked it together. He's the one that's going to be wearing it all the time, and he doesn't wear jewelry of any sort (except a watch I gave him a few years back) so I wanted to make sure it was something he liked and found comfortable.
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We got ours like 3 months out because my band was being custom made to fit around the curve of my engagement ring. You need them by the day of, so it doesn't really matter when you buy them if the ones you want are in stock. You could literally pick them up the day before if you wanted to. If they have to be manufactured or resized or anything, I would factor that into the timeline. Otherwise, it's really whenever. If you see a good sale on what you want, just buy it. It isn't like they'll spoil like flowers or something lol.
I picked my FH's out myself and surprised him with it. I already knew what he likes and his style because we've been together so long. And I got mine over a year in advance because the jeweler was having a sale and I couldn't pass it up!