What day decides the first day of your FMLA

Does your employer have to give you in writing when your first day of FMLA begins? I work two on and two off and every other weekend. My surgery was scheduled for September 16, 2013 but on Sept. 13 received a call from my doctors office stating that it had been canceled because my insurance had not approved it yet. Called my Human Resource to let them know of the situation, granted the 16th and 17th were my scheduled days to work so I was told I could come into work because I had no idea when the surgery would be back on. While at work on the 16th received a call saying the surgery was back on for the 18th which is not my scheduled day to work, now I go to Human Resource to let them know, well since I had not taken care of anything for my own personal reasons because my surgery had been canceled I asked Human Resource if I could take the 17th off for vacation so I could do this, was told to ask supervisor which he stated was fine. Went back to the office to let them know. Again the 16th and 17th my scheduled days the 18th and 1th are not, surgery on the 18th so in my mind since the 20th was my next day scheduled to work and could not physical be there this is where I started my FMLA to me but Human Resource is saying since I took a vacation day on the 17th that is where it was started, is that right? Never any communication between myself and Human Resource of the actual date that my FMLA began on. So my question is what decides the first day because by my calculations I no longer have a job because they have decided to use my vacation day a the first day.