You're buying a business or real estate. Your purchase agreement says you're going to close on a particular date. You probably won't. You'll probably extend the closing date.
Closing dates are extended because something or someone is not ready.
It could be the lender. Maybe the appraisal isn't finished on time. Maybe the loan documents aren't ready. Maybe the loan officer is on vacation.
It could be a landlord. The buyer might need a new lease or the landlord's consent to transfer the lease. The landlord probably won't put your deal at the top of its to-do list, so it might take more than a few days to get a lease or landlord consent.
It could be a vendor, supplier, or customer whose contract can't be transferred to the buyer without its consent. They might not even have a process for reviewing the request and granting the consent. They'll need time to figure it out and review and respond to the request.
It could be the lawyers. They usually are working a number of deals and might get behind on one deal as they focus on closing another. Or the legal documents might require heavy negotiations and rounds of drafting.
Deals have lots of moving parts. It's a challenge to get all the parts in place on the exact closing date contained in the purchase agreement because many of those parts are within someone else's control. In addition, unforeseen issues frequently pop up as you get closer to the closing date, and you might need more time than you expected to resolve them before closing can occur.
You shouldn't sweat it if it looks like you might miss the closing date. Work diligently to meet it, but be flexible enough to extend, if needed.