The Last Day can be celebrated doing the things you love to do with your pet, letting them enjoy the day with you and your family, and eating the things they wouldn't normally be allowed to eat.
Take photos, go to the beach, sit on the "good couch".
And when it is time to make that last visit to the vet, make sure you take time out for yourself, forgive yourself for having to make the hardest decision of your life, allow yourself to cry and grieve. They are a special part of your family and your life. It is normal to feel sad, upset, angry... even relieved.
This link is a lovely story of Duke's last day with some beautiful photos with his family.