When does labor start?

So, going off of another post about how long labor was for everyone. I just have to ask... When does labor start? What do you consider the start of your labor? 
I think I must be missing something because I would describe my labor as more than 30 hours. During that time I slept, showered, ate, went for many walks. Labor just gradually came on getting stronger. I gave birth at 2:10 am and would say that labor began in the evening of the prior day.