Days of plenty, nothing’s eves (in Portuguese dias de muito, vésperas de nada) means your days of plenty are eves of days of nothing, i.e. you alternate between splashing out and hardship, usually because you spend excessively when you have money. The saying is used as a comment on, or warning against this.
I’ve looked into this other question, which focus on wasting time or money and regretting it later. I wonder whether there is any saying that more closely conveys the idea of alternating between relative luxury and belt tightening. While the situations you use days of plenty, nothing’s eves to comment on may involve wasting money, it is not necessarily so. Take the following examples (fill the blank with a suitable saying):
They splash out as soon as they get their paycheck, and then they can hardly afford bare necessities for the rest of the month. You know, ____________
The city council spent nearly all its culture budget on free concerts in the first four months of the year, and now there won’t be any more significant events this year. You know, ____________
Especially in the second example, the city council and many among the public may think the money was well spent. But others, while agreeing the events were worth the money, would prefer the events more evenly spaced out throughout the year, and use the proverb to criticize the council’s policy.
You can off course use the saying to criticize waste of money:
He inherited quite a lot of money from his parents, but he’s frittered it all away, and now he doesn’t even have a house of his own. You know, _________________
Best Answer
Both “boom-or/and-bust and "{either} feast or famine" are pretty literal translations of your idiom, with "feast/boom" meaning "days of plenty" and "famine/bust" standing for "days of nothing."
Although "boom-or-bust" is mostly used for describing the ups and downs of whole economies and industries, I think it, and especially "famine-or-feast," would work well in your first two examples of people or governments quickly blowing through their limited budgets, resulting in them having either too many/sufficient resources or too little or no resources.
(definitions from 'Dictionary/com' and "The Free Dictionary,' respectively)
As for your third example, I'd probably go either with any of the other good answers so far that capture the alternating feel of your proverb ('rich today, poor tomorrow'/'ebb & flow'/'ups & downs') or with the good one that, although lacking perhaps the alternation, talks of the nearly-inevitable consequences of "fools [having] money."
My own late entry for your third example (and it might also fit with the first two) would also be one that doesn't directly capture the notion of alternating fortunes:
You know, all that money just burned a hole right through his pocket. What a shame.