Have opportunity to fly to headquarters for interview, what do I tell my boss?

So, Im in a great job role currently, but ive been here for 2 years. Not alot of room for upward mobility. After being passed over for a 3rd time actually for a role im currently doing btw.. I started applying for Jobs.. Well I hit the motherload of all jobs.. Its a fortune 20 company. Huge tech company, wont name drop, but I cant BELIEVE I got a first interview with them. Well now im on my 3rd round of the interview process, never dreamed id get that far, and im pretty sure I may get the job. With that being said, theyve now offered to fly me out to their headquarters in CA for an a final in person interview. I would basically miss a day and a half of work to go and do this. Im short on PTO, just had a baby so I dont have any left, and my boss is such a stickler about missing unless I have a serious illness, doctors note all that jazz. Should I just be honest and say I need to take off a day to interview with this ridiculously famously awesome company? A part of me thinks If I tell him who it is, hell understand.. Another part of me thinks it could make things really awkward say for example if I DONT get the job. On the other hand, it may make them realize how valuable I am. Im not sure im ready to start this train of "hey shes on her way out the door" thing because I know they may start treating me differently. Theres also this other part of me thats totally okay with them knowing.. They had their chance many times to promote me and I think its awesome ive been passed over and now another awesome door has opened for me.. But I havent 100% gotten then job. What should I do?