I was a single LT pinner, since I was OCS and my message came out late, like the 1st or 2nd of my promotion month. I was in Iraq at the time, and no one seemed to say a word about doing anything, so on like the 15th of the month, I was tired of wearing my only JG flight suit, and I asked my friend to just do my oath out back on top of the bunker with some black n milds and near-beer. Worked well.
Truth be told, your command will take care of you, especially if it's a big group of promoters. If not, and you're alone and unafraid on det, just get a buddy to do it or just wear the rank.
Just so you know, there are two messages: first is an ALNAV (I think?) showing the "eligible LTJG" list, and the second is a regular message showing the promotion list. The former isn't a formal declaration for promotion, the latter is.