Apparently the traditions comes from Italy (Most Argentinians are from Italian or Spanish ancestry, and our food is strongly based on them). I am not completely sure about the 29, but I think it has to do with the fact that normally we received our salaries at the beginning of the month, so but the end of the month many people are struggling because money is running out. So gnocchi, which are cheap to do, are ideal for the worst days of the months. I am thinking that perhaps it is not the 30 because many people wouldn't even have a coin to put under the dish that day.
A very interesting derivation of this is that we use "gnocchi" (it is actually spelled "ñoqui" in Spanish, by the way) as a derogative term for people who has a job but don't do it, as they are lazy and probably have connections with influential people so they won't be fired even if they don't do their jobs. The term is used specially for people who got their jobs for knowing a politician or government related person. It's very insulting. By why do we call those people "gnocchi"? Because in some jobs (sometimes in the government bureaucracy) salaries are actually paid on the 29 of each month (unlike what I told you above). And those people are said that nor even go to their works...except the 29 for getting their paycheck!
(of course, that was long before salaries were just transferred to a bank account)
