UPDATE calendar c1
INNER JOIN Customer c2 ON c1.SeatingID = c2.SeatingID AND c1.BusID = c2.BusID
SET c1.Customer = c2.ID --or SET c1.Customer = c2.PassengerName or whatever you want.

UPDATE calendar cal, customer cust
SET cal.Customer = cust.ID
where cal.SeatingID = cust.SeatingID
and cal.BusID = cust.BusID
and cal.DATE = cust.DateOfTravel;

update calendar ca left join customer c
on c.DateofTravel=ca.Date and c.SeatingID=ca.SeatingID and c.BusID=ca.BusID
set
ca.Customer=c.ID;