Open Question: What are some good excuses to call into work?

ok so here's the deal. Im a hard worker and a trustworthy employee. But then i have a sister that i havent seen in a little over a year because neither of us can travel hundreds miles to visit because of our budget. yet her job is paying for her to come to my town for a day to do a workshop. we found out at the last minute, i want to spend the whole day with her, but i have to work. i would llike to call in and let them know the truth but they totaly wont understand. so does anybody have any ideas? It's just that i've never called into a job before. oh and by the way i work at a retirement home.