Need some insight please!

Hey ladies,
​So ive a question. My husband and I currently dont live in the same country but we are hoping he can get a visa and come for the delivery. He wont be able to get more than 6 weeks off work so I was just wondering..when should we book his ticket? Like how many days prior to my due date? Its my first what are the chances of being early or late? I was my moms first child and I was born a week late. 
​Im due in early June, so need to plan fast. If I book his ticket for like a day before my due date would that be cutting it too close? Please advise!