I came out to my parents the day after I started on HRT.
It was some pretty strange timing, because my sister had told my parents that she was gay the week before (I had known for a while but hadn't felt it was my place to say anything, as I'm sure my sister felt about me, she kind of knew about me, even though I hadn't given her the whole story), but that helped me so much in being able to just stand up (figuratively speaking) and tell them exactly what was what. I had a month in between arranging the appointment where I would get my prescriptions and the actual date, and for the three weeks prior to my sister beating me in the first-sibling-to-come-out race, I was absolutely terrified about doing it. Having my sister lead by example was the best thing that could have happened. I realised that if she had the courage to tell the people she loved exactly who she was, then I had better work it up and fast. I was planning to tell them the next time I saw them after I started on HRT, but because they live a fairly long way away and it wasn't looking like I'd have a chance to see them for at least a month or two, I ended up calling them on the phone and telling them exactly what I felt and what I would be doing about it. I didn't really want to do it like that but I thought it would be worse if I started on HRT and went a few months before I saw them, having really no idea at that point just how much my appearance might change in the interim.
They took it very differently. My mother, though initially a bit shocked, was brilliant, and by the end of the phone call told me that if I needed to do this to be who I really am, then she would support me. I talk to her by phone all the time about everything that is going on, I never expected her to be so understanding of everything that I'm going through. My father initially thought that I had been pressured into transitioning by my therapist, and after explaining that this was something that I had felt for a very long time and had very carefully considered whether it was something I should be doing or not, he didn't speak to me for about 2 months. After that we were back on normal speaking terms, but he hasn't really wanted to talk about my transition at all until very recently. He's promised to sit down with me next time I go home to visit and let me talk through everything with him, which probably wont be until christmas now, but its progress all the same.