many thanks for this. I had managed to work some of that out already, but my dark, suspicious nature makes me believe there is something else being referred to here. The context, from Sprünglin's memoirs (he was Swiss by birth and had just entered French service in 1806, though the meoirs were being written in 1836) is from a period in early 1808 when he is a captain at the depot of the 45th ligne at Liège:
"Depuis qu'au siège de Dantzig j'avais l'occasion de me distinguer, que mon nom avait été cité dans le Moniteur, que j'avais obtenu la croix et qu'il m'était permit d'espérer quelques chances plus heureuses pour mon avenir, M. Bay s'était un peu radouci et avais permis que Louise rentrât en correspondance avec moi. Le Geldstag avait eu lieu, et il ne me restait plus d'espoir de rentrer dans ma patrie, si mes affaires ne pouvaient s'arranger dans un avenir plus ou moins éloigné. Ce sentiment cruel empoissonait ma vie et me donnait une morosité et une tristesse habituelles qui me rendait peu propore à jouir de la position douce et tranquille que me donnait ma position au dépôt et le bon accueil de la société à Liège."
Bay was Sprünglin's father-in-law, with whom he had fallen out badly over political differences, leading to his self-imposed exile from Switzerland in 1806 to seek employment in the French army, subsequent to Napoleon's edict allowing foreign officers to enter French service in the same rank they had had in foreign service, even if they had fought against France.
From the context above, I feel as if Sprünglin is referring to some event - maybe another decree or edict - that made it more probelmatic that he should eventually be able to re-enter Switzerland. This seems to me to be more important, at least to Sprünglin, than the day on which taxes were due, or similar. So I am wondering whether some political event with regard to Swiss relations with the French empire took place at that time, which was known as Geldstag to contemporaries?
Curiouser and curiouser...