Is there a best time to rollover 401k from previous employer to current employer?

This sort of direct transfer involves zero tax issues. The selling of your investments within the retirement account, and the subsequent purchase of investments in the new account will not have any tax impacts.

There can be a gap where your money isn't invested in the market while the funds are being transferred, but the time frame should not be more than a few days. Ask the two 401k companies how long it will take.

If you are offered the option to have the funds sent to you and then you send the check to the new provider, pick another option. The direct transfer is faster, and you don't have to worry about deadlines, or who the check has to be made out to.

The new company should keep the flavor of the money. So Roth 401(k) funds should still be Roth 401(k) funds; pre-tax contributions should still be pre-tax contributions...

If you have invested in your old companies stock, you will probably not be able to keep that investment, unless the new plan allows you to pick individual stocks.

Sometimes the new company allows to to pick your investment options before the funds arrive, others put everything in the default fund, and then let you divvy it up after the transfer. Don't forget to do this step.

There is no reason to wait for a high or a low for most of you funds. You should be moving into similar funds so there is no time that is better than any other. The exception would be if you have a large amount of company stock and it would be viewed as a bad decision to sell now. That will depend on the company. Otherwise there is no reason to wait.