According to Variety.com
"In what is looking to be a bountiful box office transformation, DreamWorks and Paramount's "Transformers" made north of $8.1 million in its early debut Monday evening on 3000 screens, beginning with the 8 p.m. showing. Figure could be higher than $8.5 million."
Pirates 3 earned $13 million when it took a similar approach Memorial Day weekend, and ended up earning $140 million over the four day weekend. Hopefully Transformers will go the distance.