Serengeti National Park

From the hotel website
https://www.fourseasons.com/serengeti/getting-here/

Planning your international flight connections
Tanzania is serviced by a number of international airlines, with Kilimanjaro International Airport being the closest airport for connection to the Serengeti, yet Dar Es Salaam and Zanzibar also providing alternative options. From each international airport, you will need to take a domestic flight into the Serengeti National Park. Depending on the time of your international flight arrival, it may be necessary to overnight in Arusha, Dar Es Salaam or Zanzibar, prior to flying to the Serengeti the next day.

Hi Carol, I don't know the Four Seasons, but I know that area of Africa reasonably well.
After reading the details from the hotel, as mentioned, Kilimanjaro airport is the closest...and the airport typically used for safaris on the 'northern circuit'. Dar and Zanzibar are quite some distance, but do have the occasional flight to the Serengeti. Keep in mind, the Serengeti planes are maybe 16 seats and from the last 2 cities, will be a lot of up and down. Consider it often the milk run.
Arusha is still an hour west of the airport. I don't know the best hotel in Arusha (non are the Four seasons equivalent), but take the best in town and arrange them to book a driver for you. Taxis are notorious for overcharging. Or, at the least, find out what the current rate should be.
From Canada, I tended to use Air France or KLM. I see Delta codeshares with KLM. As it is 24 hours in transit, I might recommend a day or two layover in Amsterdam before heading to Kilimanjaro (JRO). This flight does arrive at night, but not super late, so you do want to have a ride arranged.
You need to decide whether you can fly from JRO to Serengeti, as most local flights leave from Arusha (ARK) airport. If you can leave the next morning from JRO, then stay nearby. Otherwise go right into town as ARK is west of Arusha town.
If you don't mind a milk run flight, Dar might be 'tidier' for an international flight, however, traffic can be stressful for ensuring getting to the airport the next morning.
Zanzibar could be an interesting add on to this trip.

If JRO doesn't work for some reason, the other big airport in the region is Nairobi (NBO) from where it will be a 4-5 hr transfer just to Arusha and will involve getting multiple visas. If this is the case, then DAR should work.

Stating the obvious but do make sure you plan for your yellow fever shots if you don't have it already

yellow fever and anti-malarial meds.

According to my excellent travel clinic, Tanzania does NOT require a yellow fever shot at this time unless you travel to certain countries first. Check the CDC website for up-to-date info.

Yes, you need anti-malaria drugs.

Things may have changed on the yellow fever cert. Double check else you may need to get a shot when entering the country or be turned back. I know last August I was asked for it when entering Tanzania, Uganda and Kenya - only Rwanda didn't ask for it.

There is always a debate on the yellow fever issue. One needs to remember it is often not for your sake, it is to avoid you carrying it into their country and they can determine what they want. Zanzibar in particular is stringent that you have it.
Now the flip side, is that in the case of you not being able to have the shot, you can carry a letter that exempts you from the immunization. This is generated by your travel health care physician.
And like anything, not every doctor has the same 'opinion' or enacts the recommendations the same.
You as the traveler, however, have to decide whether or not you are prepared to be barred at the border. Because it is ultiimately how the guy at the gate is feeling that day.

The advice from staff at the Tanzanian embassy in D.C. and despite asking a second time, we were told unambiguously to arrive in their country from Kenya with evidence of the yellow fever vaccine. Period. However, no one checked when we entered Tanzania.