That's one of the toughest things to do...
If it's not suffering, hang out with it as much as you can today and take it in later, have the whole family make a fuss over it... Give it a good farewell....
If it is suffering, get the family to say their goodbyes and take it in to put it out of its misery...
You know it's the right thing to do... You gotta do it, be strong....
Look on the bright side, you gave it a good home and you and the family have many good times and memories... All the joy that it brought you...
The worst part of owning pets is letting them go at the end....
When I had to put my dog down, I stopped at the sub shop and got him a large milk shake and he ate 3/4 of it... He loved milk shakes, I hope it helped comfort him... (I think cats like vanilla milk shakes also - nudge, nudge....)