This article from Variety talks about WALL-E's opening day, which, as the title of this post states, was Pixar's best opening day ever! It was shown in about 4,000 theaters and made around $23.1 million domestically. The next best for Pixar was The Incredibles at $20.5 million on opening day.
WALL-E also broke another opening day record! The most lucrative opening day for a G-rated movie was held by Finding Nemo at $20.2 million, but WALL-E beat that by about $3 million. WALL-E is now on its way to beating Pixar's best opening weekend, which is currently held by The Incredibles at $70.5 million over 3 days.
Another record that WALL-E had potential to beat but didn't was the opening day for an animated feature. WALL-E came in third place after Shrek the Third ($38.4 million) and The Simpsons Movie ($30.8 million).