Read about subjects here; then let us know what you think and why. Someone here will be happy to let you know if you're right.
Or, ask yourself this question: WHAT is this talking about? That should give you the subject.
The subject of this statement is a specific day that is being linked to an old Roman festival. It is unclear what the day or festival in question is based on the given information alone. To determine the subject, we would need more details or context about the specific day and festival being referred to.
