This website is not connected to one specific location. So we would not know which Willow Elementary school you are referring to. I googled this and found many Willow Schools around the US.
Usually, in front of the school, there is something posting the start date of school. If not, perhaps you can google the school and click on the link. I am sure you will find it. The link below will take you to a Google search.
Of that does not work, go to the school. Surely there are administrators there.
Call the district office and they can tell you the date for opening of the schools. Usually all schools within a district begin on the same date. I will safely assume that you must be new to the school or you wouldn't be asking this? And, if so, when you registered your child, they should have provided you with a school district calendar with all important dates listed or, one will be mailed to you about a week prior to the school's opening. Another option is to visit your district's website!