Well, this is kind of late, but, no, you can not learn to tune a piano in a day.
Why do I say this? Well, I am a Registered Piano Tuner with the Piano Technicians Guild and I've been teaching piano tuning for eight years.
You can however learn a lot in a 20 hour basic crash course. But how well your tuning will sound after depends on your ear and aptitude.
My student's success rates, after 20 hours of instruction and supervised practice, are roughly:
Have a very poor understanding: 30%
Can finish the tuning but the piano sounds horrible: 30%
The final tuning is not great, but playable. Sounds like a piano that hasn't been tuned in a year: 30%
These students have an exceptional understanding of the concepts presented, and their final tunings are impressive: 10%
Potential of Students.
Percentage of students who can achieve professional results with practice: 25%
Percentage of students who could probably learn to tune their piano to their own standards: 50% (includes the previous 25%)
These statistics are subjective, from my experience teaching over 200 people to tune their own pianos.
You can break strings and round off the edges of the tuning pins with poor technique and poor tools.
You can drop the tuning hammer and chip the keys.
There is other damage that could be done just by being careless.
The Reblitz book is not great for tuning, IMHO.
Hope that answers your question.