As one last example for this series, I will use that of a family going through a defined season of food restriction. This could be for any number of reasons including weight loss, spiritual reasons (fasting, Lent, etc.), and so many more. It may be in the form of the GAPS diet, or the like; weight-loss diets; juicing; etc.
In these cases, it may be best to respond to a personal, casual invitation by politely declining. You could even offer to merely post-pone the visit.
“…Thank you so much for the invitation. Due to some temporary complications, we think it best to decline. But, we would love to find another time to get together! After the end of the month, our schedule should be much more open…”
The best thing to keep in mind in ALL of this, is how you can best serve others. This goes for hosts and guests alike. Again, as a good foundation to all social behavior, remember Romans 12:10.
Be kindly affectioned one to another with brotherly love; in honour preferring one another.
Is that not the entire basis for manners/etiquette in general? If you always remember to prefer one another in honor, I think basic manners will come quite naturally.