The absence of an article tells you "market-day" is something that (usually) happens at some (reasonably) regular interval -- once a week, once a month, every three months, etc. The sentence means "This is the day that I regularly go to the market," rather than "This is a day on which I have decided to go to the market." For example, in some homes Monday is laundry day, Wednesday is market day, and so on. In that case, if you woke up on Friday and realized you needed groceries, even though it's not your usual day for going to the market, you would include the article and say, "It's a market day today." I hope that helps!