What does the pronoun "it" refer to in this context?

Native speaker’s answer
Rebecca
In this context, the word "it" is used to identify that the day is grandma's birthday. The same way you say what day it is in the week. The word "it" emphasises the subject "grandma" and object "birthday" later in the sentence. Ex: It's Friday today! Ex: What do you want to do today? It's a public holiday!