Have you ever met someone named Glinglin? Or even heard of a saint called Glinglin? Probably not. Here is the thing: there is no Saint Glinglin's day on the calendar, and no one ever named their newborn Glinglin - or so we hope. So, if someone promised for anything to happen on the day of Saint Glinglin, you may be waiting a while for that day to come around...
In everyday French, à la Saint-Glinglin means there is no way to know when. Most often in fact, it means never, as in when pigs fly and hell freezes over. It is similar to the French quand les poules auront des dents - when(ever) hens have (grow) teeth, with the slight difference that Glinglin means probably never, as opposed to absolutely never.
The expression is the result of a semantic shift from seing to saint. In old French, a seing meant "signature", as well as "signal" or "church bell." Since both seing and saint are homonyms - i.e. pronounced in the same way - and both are church-related, it is easy to see how the somewhat scholar word seing would be replaced in popular French with the more familiar word saint. As part of France’s Catholic heritage, every day of the calendar is associated with the name of a saint. In the past, it was quite common to use a saint's name rather than a number to indicate a date. For example, one would say à la Saint Valentin instead of February 15, or à la Sainte Catherine for November 25.
As for the famous Glinglin, who never existed, it came from "glinguer," a dialect form of Eastern France, itself derived from the German "Klingen," and meaning to ring, to resonate.
The original expression à la seing glingue was therefore equivalent to "when the bell rings." In the not so distant past in Europe, church bells used to ring every hour to tell the time. They would also ring for the various religious services throughout the day in addition to weddings, funerals and other special occasions. To propose to pay someone on Saint Glingin's day was to propose to the ignorant who does not know that Glinglin was never beatified, to pay at a bell ringing, without specifying which one. That could take a long time.
Il me remboursera à la Saint-Glinglin!
He’ll pay me back when hell freezes over!
J'ai l'impression que ma chef me donnera mon augmentation à la Saint-Glinglin.
I have a feeling my boss will give me a raise when pigs can fly.