When declining an invitation, it isn't mandatory that you give a reason why you can't attend. In the case of an informal reply to a well-known acquaintance, however, it is a courtesy to do so.

Letter to Decline an Invitation

(Date)

Dear Dennis and Ann,

Unfortunately, we won't be able to attend this year's event. Alex's parents will be spending that weekend with us. They're stopping by on their way to Florida, leaving earlier than usual so that we can attend an out-of-town family reunion on Saturday. We'll be spending the weekend with relatives, and not returning home until Monday.

I'll drop off a jar of my bread and butter pickles sometime before we leave. Let us know if there's anything else you need, too.

Know that we'll be with you in spirit, even though we can't be there in person.

Regretfully,

Alex and Amanda