If this was a preplanned rendezvous to meet someone unkown, I doubt that Libby would have asked for Kelsi to accompany them.
She did ask, and Kelsi declined in favor of spending time with her boyfriend. Kelsi has spoken about this with some regret.
JMO
I think this was in regard to the previous day. Kelsi was going out with her boyfriend and friends and Libby asked if they could go too. Kelsi has spoken of her regret in not agreeing to this, but that at the time she didn't want her kid sister hanging around with them. On the day of the murder, if I remember correctly, Kelsie dropped them off en route to her visiting her boyfriend and then going to work.