For me it's the beginning part. I'm always so nervous going into dates, like when we first meet up with each other. I worry about it being awkward because of my shyness, or because of our personalities just not clicking.
As the date goes on I usually feel a lot more comfortable with the guy and less nervous.
I guess it mainly happens before the date, when I'm worrying about looking good and creating a good first impression. I also tend to get nervous about conversation flow, as I'm so, so bad with awkward silences... I just don't know how to deal with them!