Well, for many of us, the answer when to go to secondary is "never" as there are some new schools of thought on that. I'm an old winemaker, and I always used to go to secondary like clockwork, but most people (including experts) are saying that it's not only not necessary, but may be detrimental.
So you won't get an answer on "go to secondary on day _____" because it's not recommended.
But if you choose to do it, then do it ONLY after FG is reached. The SG shouldn't change in the secondary at all. It's a clearing vessel. In a commercial brewing, that clearing vessel is called a "bright tank". The beer is finished, but they want to use the primary fermenter so it goes to the bright tank to clear up a bit. You may see some sediment in secondary, but not a drop in SG. You can keg/bottle any time you want- I prefer to do it after the beer has cleared.
I usually leave the beer in the fermenter for 2-3 weeks and then rack into the keg when the beer is clear.