flightaware.com/live/flight/COA551
This flight has been Houston to Seattle at night for quite some time… then it takes a day off from that and is Newark to the Caribbean? What gives?
P
Could be a schedule change. That flight from IAH to SEA may be discontinued or changed to a new time and flight number, and the flight number is recycled to the EWR flight.
On some regional airline flights you may find multiple flights under the same number because they use a 4 digit flight number but their callsign uses 3 digits. I don’t have a real life example, but take American Eagle, they may have a 3999 and a 4999, and the ATC callsign may be EGF999 for the different flights. This probably isn’t the case for CO though.
Thats my guess.
But I have tickets on that route for that flight number in a month… I just thought someone here might be able to shed some light on it. 
I’d check with Continental.
As pointed out by pfp217, airlines do change flight numbers.
Airlines do quite a few once a week trips to leisure destinations on Saturdays, but I as far as I know most of them don’t use the same flight number that another route uses the other six days.
While we’re talking flight numbers, Skywest really messes with you…they use the same flight number for the whole roundtrip.
SKW6257
Some airlines also use one flight number for one routing most of the days of the week and another routing for other days.
Many overseas flights used to do this. A great example is Pan Am 811. It was a flight from Los Angeles to Honolulu to Sydney. Some days the route between Honolulu and Sydney would go via Nadi, Fiji while other days it would stop in Pago Pago, American Samoa.
After Sydney, the flight continued to Hong Kong. Between Sydney and Hong Kong was at least one stop which varied according to the day of the week.
The round-the-world flights of Pan Am, TWA, and British similarly operated this way.
A few air airlines currently have flights that will, for example, six days a week on a routing of A-B-C but on the seventh day may operate A-B-C-D.
and that might the most probable in this case.
and that might the most probable in this case.
edited to remove double post…sorry