Ideal Time? --> ASAP

Just do the math.

If dividend distributions on VTSAX are are about 2%, and you catch 2 (max) quarterly distributions in the next 4 months, then you will owe taxes on the 1% dividends that you received.

Also known as [your current top marginal tax rate x 1%].

So if you are in the 22% tax bracket and invested 100K, you would owe an extra 220$. Considering Mr. Market can rise much more than 1/5 of 1% on any given day, much less any given 1-2 month period, I would consider the above pocket change.

I'd also note that if you lose (say 3%, in the next couple months), you can tax lost harvest and then get 3K x .22 = $660 in tax benefits next time you file in April 2020.

Check my math,

All the best,

JGS