There's never a best day. I believe there are two parts here: when to reinvest the dividend and when to rebalance.
My take is that for reinvesting the dividend, I'll add it to the asset class that is lagging when there's a decent dollar amount. For example, if I only received a $100 dividend, I wouldn't immediately purchase, but once I accumulate $1000, then I'd consider purchasing. At least, that's the idea on this one; I haven't actually done it yet, so I don't know if I'll stick with $1000 or some other value, but I think the overall idea will stay the same.
As for when to rebalance, this is something I will do when my asset allocations are a certain percentage out of whack. For example, if I'm going for 40/40/20 and if it goes to 45/37/18, I'd probably readjust, but if its 42/39/19, then most likely not.
Btw, I'm new to the board, but have been lurking here for awhile. I'm still in the process of tweaking my AA and at this point, figuring out the logistics of maintaining the AA between different accounts.
I've really appreciated the content on the forums so far - it's been a great help in getting things going.