Each day, a player can perform one action. Actions are normally work related, but can also be other things such as travel.
Out of date - there are now actions that no longer take a full day, and thus more than one can be done in a day. Examples include shorter shifts of mine work or of herb gathering. Travelling and working for the town militia also do not take a full day, instead, whenever you start them, they take until reset at 3:00am GMT. This means that if you are able to be logged into the game near reset and have finished your other tasks, you can do a full "day" of travel or do a full shift in the militia in the bit of time that is left.