Say you had planned to go somewhere about an hour's drive away for the day that was nowhere special, no special event, no tickets etc. ie. you could go there any time.
Say your DS and DIL ran you in the morning and invited you over for the day to play with the DGCs (also about an hour's drive) as they (DS and DIL) had a rare free weekend day (as DS often works weekends).
Say you only saw them once every 6-8 weeks and you professed that you loved them a lot and loved spending time with them, and you complained about how little you did get a chance to see them.
AIBU to think that most people would grab the chance and say 'hey ho, we'll go where we planned to another weekend - we'll be with you in a couple of hours'?
My PIL bemuse me! 