When do we send K-1's to the beneficiaries for my dad's estate (he died Aug, 2019), and do we send them to the IRS?

The estate is required to send forms K-1 to each beneficiary by April 15, 2020.

All copies of forms K-1 must be attached to the estate tax return (form 1041) which is due by April 15, 2020.

Please read this TurboTax article for more information.