According to the official Schulz timeline at the Charles M. Schulz Museum site, today was the first PEANUTS daily. The strip debuted in seven newspapers on October 2, 1950. Other "this day in history" type sites say that the strip initially appeared in nine newspapers, but I'll place my trust in the Schulz Museum's site. He made $90 for the first month's worth of comic strips.
Above: Charlie Brown's debut, 57 years ago.