Today is Terminalia, the festival to the god Terminus, though he has never had a form beyond the boundary rock, and he has no personification or myth associated with him. He is one of the more pure forms of Roman deity in that he was never made to change into the Greek forms with a myth, history, and rules attached.
Today, owners of adjacent property would crown his statue, a stone or post in the ground to mark the boundary line, with garlands and erect a temporary alter on which they would offer honey, wine, sacrificial lamb, or other deserving items to his pleasure. They would sing his praises as well. There was also a public celebration which took place around the sixth milestone marker on the road to Laurentum. It is assumed this is because the sixth milestone marker was the original boundary of Roman territory.
So today is a day of celebration of markers, boundaries and neighbors. It is when you rejoice in having good clear ends to things. This was also the end of the year in the Republican calendar.