It's a particle - in this sentence it sort of acts as the " 's" in "today's". The meaning of this particle (like most others) changes depending on context. It's often interpreted as "belonging to" (e.g. my friend's dog = 友達の犬/ともだちのいぬ), or can be used to indicate the position of something in relation to something else (e.g. behind me = 私の後/わたしのうしろ).