This classic summer cocktail, celebrated every year in, er, February, is all about the tequila, right? Some French liqueur makers would like to change your mind, with a free drink…

The natural reaction to most spurious-sounding “national this or that days” is to ignore them, but we’ll forgive 22 February’s National Margarita Day (even though the nation in question is neither Mexico nor the UK but, you guessed it, the US). Why aren’t we sceptical? Because who can object to an occasion designed specifically to encourage shaking the ultimate summer drink on a winter’s day in order to bring a bit of light to the season? And when it involves a free drink served by one of the best bars in London, even better.

On Thursday 22 February, Nightjar will be pouring 100 free grand margarita cocktails for guests (50 each at its Old Street and Carnaby Street bars) – first booked, first served – as part of a partnership with Grand Marnier orange liqueur. The classic margarita recipe usually lists triple sec as the third and last ingredient after tequila and lime. And that often means little attention is paid to the quality of what should be a crucial modifier.

Grand Marnier is different to a triple sec, many of which use artificial orange flavouring infused into a neutral spirit of dubious origin. Grand Marnier is a blend of cognac – 51 per cent in the classic Cordon Rouge bottle, and over 90 per cent in prestige versions such as Révélation – with a bitter orange essence. It’s much more rounded and layered than triple sec, with spice, nuttiness and a variety of fruit notes, not just the green peel of the bigaradia orange.

This all adds complexity to the margarita, with an earthy depth and less zing on the citrus side, making it a more comforting year-round cocktail rather than just a summer refresher. In fact, the recipe for a grand margarita is 3:3:2 parts ratio of tequila, Grand Marnier and lime juice (having also tested it at 3:2:2 and 3:1:2, Brummell would agree it is the best ratio).

Nightjar is adding its own twist with a drop of tincture based on the zara cocktail (smoky whisky, maraschino liqueur, absinthe, bitters and lemon zest). Also on the menu are cocktails called passion roja (Grand Marnier Cordon Rouge, Campari, Espolòn tequila, black cardamom and lime) and a serious indulgence, at £26, called a honeymoon, which features Grand Marnier’s Cuvée Louis Alexandre (82 per cent VSOP cognac) mixed with Johnnie Walker Blue, geranium syrup, Nightjar’s forbidden fruit liqueur, mead and lemon. These, and the Nightjar grand margarita, will be on the menu until the end of February.

grandmarnier.com; barnightjar.com