"Ĝi" is used here because it is a definite thing that canbe switched with a regular noun, but if the sentence was talking about how it seems like a long day, then it would be "Estas tre longa tago". We do not really know what that "it" is, such as saying "It is raining" in English. What does the "it" represent? I don't know. We just use "it" as a placeholder for the subject since "Is raining" or "Rains" would sound weird and be wrong, but in Esperanto, you would just say "Pluvas", literally "Is raining/Rains" since it is fine to do it and using "it" is unnecessary. Hope that explains it.