Is there any English equivalent to the Portuguese proverb “days of plenty, nothing’s eve?”
Solution 1:
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.
Boom-and-bust
adjective 1. characteristic of a period of economic prosperity followed by a depression.
Also, boom-or-bustfeast-or-famine
adjective 1.characterized by alternating, extremely high and low degrees of prosperity, success, volume of business, etc.: "artists who lead a feast-or-famine life."
(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:
burn a hole in/through {somebody's} pocket
If money is burning a hole in your pocket, you are very eager to spend it.
(from 'Cambridge Dictionaries Online')
You know, all that money just burned a hole right through his pocket. What a shame.
Solution 2:
I'd suggest, the wheel of fortune is always in motion and rich today, poor tomorrow
People's fortunes are constantly changing - somebody who has good luck one year may have bad luck the next, and vice versa. It seemed that nothing could go wrong, but the wheel of fortune is forever in motion, and disaster struck the following day. The proverb was first recorded in 1748 in Papers of Benjamin Franklin. The Facts on Files Dictionary of Proverbs
fortune is fickle might also fit the bill. Ngram
Solution 3:
If someone spends money extravagantly without any forethought, only to regret later, the I'd say
A fool and his money are soon parted
something that you say which means that stupid people spend money without thinking about it enough
[The Free Dictionary]
Usage
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, fools and their money are soon parted.
Or
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, a fool and his money are soon parted
Solution 4:
1) what goes around comes around from OD:
proverb: The consequences of one’s actions will have to be dealt with eventually.
A couple more suggestions :
2) Ebb and flow from TFD
... You know, their bank balance ebbs and flows like the tide.
... You know, that's the type of ebb and flow that event goers have to endure.
3) Or on similar vein, ups and downs from reference.com.
rises and falls of fortune; good and bad times: Every business has its ups and downs.