Well since the term work dress tends to be a modern label and even day dress in the period is more commonly referred to as a "dress" it's hard to find primary documentation for these differences. My personal definitions:
Work Dress: Something I am going to wear and get actually dirty (Please keep in mind I do 1850-1860 events). My work dresses are normally made of reproduction cotton, with gathered bodices and gauged skirts, They tend to be hemmed to the bottom of my ankle bone (shorter than my normal dresses). They have very little decoration other than a possible sleeve cap or directional use of fabric and I also tend to wear them with a neckerchief rather than a collar. On my head I wear a slat bonnet. I'd use a work dress for camping and walking the Mormon/Oregon Trail or reenactments for such purpose, doing demonstrations of laundry, gardening, etc. Heres a recent picture of one of my latest work dresses.
Day Dress: Something that I am going to wear to be "seen" in. My day dresses are made of tropical weight wool or reproduction cotton. They have gathered, pleated or darted bodices depending on fabric and sometimes gauged or pleated skirts. They are hemmed to about 2" above the floor and I wear them with collar and sometimes cuffs. I also add accessories and either a sheer corded sunbonnet or a fashion bonnet. I wear my day dresses for instructional presentations in classroom settings, attending church, visiting situations or events where I am sewing or doing other "not dirty" work. Here's the same dress with the tuck to shorten the skirt let down and what I consider "day dress" accessories.