Does it matter when I purchase…as far as a taxable account goes?

Tax advantaged accounts are maximized.

Not sure how a taxable account works as far as not cashing out, only buying. Can this be done at any time? Then it’s just taxes on the dividends only since I didn’t cash anything out?

Does it matter when I purchase? As far as the following tax year goes…is buying in January, or June or December the same thing?

Thanks for any help