I'd agree with bex regarding LHR longhaul.
A big chunk of the passengers are on Business travel, which isn't that seasonal, a lot of flights are too/from the southern hemisphere so different holidays, different seasons come onto play, plus what might have been regarded as off season once upon a time is now when those with flexibility try and travel in the hope of cheaper fares.
End result is any monthly variability you do notice won't be a lot and tends to be down manpower fluctuations on fleets and the odd outlier like engine issues taking work from one fleet and lumping it on other.