I used to always get dressed up for the day immediately. Since I've become pregnant and self-employed, I've found myself typically lounging in my husband's oversized clothes most of the day... mainly because that's all the fits anymore. =P
A personal day is determined by what you do with your day. Reading a book, lounging in front of the tv, going for a bike ride, painting my nails, coloring my hair, wearing comfortable clothes, anything that brings a little enjoyment. A personal day does not have to do with work. Wearing you PJ's when working, as I work from home, might make you feel alright. However, it makes me slow and more sleepy. Wearing jeans and a comfortable t-shirt help my speed and attention.