Lachey got things under way Friday with a Twitter message to his fans promising to leave his 30s with a bang.
"Well, the 40th birthday weekend has officially begun! Pray for me.... Gonna be a big one!," he wrote.
However, the singer-actor told People magazine he was in the dark about details.
His wife Vanessa, who turned 33 Saturday, was in charge of the planning and execution.
"She has taken the 40-year-old birthday baton and run with it and I'm sure it's going to be an amazing night," he predicted.