I don't know how you potty train in a day, but I'll tell you my success with both kids. Both were potty trained in a week, DS7 was 21months, DS4 was 27months when they were done training. Here's what I did:
1. No pull-ups - underwear or nothing. They got to go and buy pick their own underwear. During the day at home, they either went free range or wear underwear.
2. Give them plenty of drinks
3. Take them to the potty every hour on the hour. No exceptions, even when you're out and about or eating out. Sit them on the potty for at least 5 minutes. If they don't need to go, that's OK, let them play again, then take them to the potty again the next hour.
4. No more drinks 2 hours before bedtime. If they want drinks, then just water. No more water at all 1 hour before bedtime.
5. Before bedtime, go to the potty one more time. After they're sleeping in bed, and before you go to bed, wake them up and take them potty again. Don't turn on any lights (a night light in the bathroom works well here). If they're too sleepy to walk to the bathroom, carry them, sit them down on the potty and wait 5-10 minutes.
6. When they wake up first thing in the morning, have them sit on the potty until they go.
7. Repeat again and again. Consistency is key here.
8. Accidents will happen. Don't yell at them when that happens. Just have them sit on the potty while you clean up the mess. When you're done cleaning, let them know that the potty is where it needs to go. If they have a number 2 accident in their underwear, clean the underwear and if possible, dump #2 in the potty, while telling them that the potty is where said items must go.
9. Lots of praises and hugs and rewards. If they go dry during the night, bigger rewards. If they still have problems during the night, place a waterproof mattress pad.
Just keep at it. I haven't seen an adult yet that's still wearing a diaper because they weren't potty trained.