Finally got a new job! When is the best time to give notice if we're talking about a mid-January start date?

After searching for a while, I've finally landed a job. Big step up for me in terms of responsibility and pay. The new company is clearly very interested in having me come on board, which made negotiation a breeze. I accepted the offer on 12/3. We agreed on a start date of 1/18 because HR said they usually take a month or so to get the onboarding process done. I'm still going through my background check (thanks for nothing hireright). I hope that will be concluded early next week - criminal history is expected tomorrow and I finished my drug test on the 8th so maybe tomorrow as well.

They have had me setup a HR portal account already, hiring manager has been in touch and let me know that they're planning on shipping hardware to me shortly. The only thing I'm waiting on is for the background check to clear. I'm replacing a person who is retiring so they need the position to be filled for sure.

My current employer is going through a restructuring - my manager has seen his team grow and wants to put me in charge of a group. Sadly that increase in responsibility comes with no pay hike, which is why I'm leaving. I really like my manager and we have a great relationship.

So my question is this: For a start date of 1/18, I would have to put in my notice on 1/4 - basically the day everyone returns from vacation. It would catch my manager off guard and leave them scrambling. Assuming my new employer tells me that my background check looks OK early next week, should I at least mention to my manager before we go on vacation that I've accepted another offer and will put in my 2 weeks after we return? I figured that way they can strategize and work the team structure. But if something happens to my new offer in the next month, I will lose the title bump because my manager's plan would have changed.

Any thoughts on what to do?

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