Could some find out who bought a stock?

Say... someone wanted to find out who bought a particular microcap stock at some specific date... how would someone go about doing that?
A court issued subpoena?


Thanks

All I know is that the SEC makes mutual funds publish their holdings.

The SEC requires a filing for any holding (or intent to obtain one) in excess of 5% in a company's stock. Beyond that one would have to go through broker records in the case of most individual traders.