I don't want to ruin his day
Your day is ruined isn't it. But that's okay? He's been found lying to you, and you don't want to ruin HIS day.
If thats how you feel about it, let it go. Act like you didn't see anything and go on with your week and your great plans.
Just be aware that it is YOUR decision to rugsweep this. IMO you drew a line in the sand, and he has blatently stepped over it.