Welcome to the Event, jfacciolla!
The IRS expects you to pay taxes as you earn or receive income. If you fail to do so and your tax bill is more than $1000, you may face penalties.
As you rightly note, you won't know what your total capital gains or losses will be until the year's end. You can avoid the penalties by making sure your withholding and estimated tax payments equal at least 100% of the total tax you paid in the previous tax year if your income was under $150,000. If over that, payments and withholding should equal at least 110% of last year's taxes.
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