Night and evening may be interchangable in English, but English is a very loose language.
Czech and other languages are very exact in their own ways.
That's what makes English easier to mash a bunch of words together, out of order, and still be understood. Even German (same roots as English) behind nonsense if not in the correct order or a similar word is substituted
To exemplify what CultureSharked said with this particular case:
To greet somebody in the later parts of the day (no matter how late into the evening/night), always use "dobrý večer".
Use "dobrou noc" (literally - good night) to wish someone a good sleep or when departing in the later parts of the day.
"Dobrý večer" is a greeting. "Dobrou noc" is used when departing and/or wishing someone a good night's sleep.
Dobrý večer is just a greeting, it is really formalized, not used as an actual wish. Unlike "Hezký víkend", this one is not used to greet a person when meeting or departing, it is really a wish to enjoy the weekend. Often connected with some formal farewell. Ahoj a hezký víkend! Bye and have a nice weekend!
The natives know which diacritic marks belong where so they often omit them themselves in informal e-mails, internet discussions, facebook and so on. However, diacritics should be used in newspapers, books, official letters, documents AND when learning the language - because without the marks you do not see how to pronounce the word if you do not know it well.
except your native wife may not have realized that "dobrý večer" is a greeting when meeting s.o., whereas "good night" (space or not) is said when one is taking leave. hence you cannot mix them. however, when not used as a greeting, "večer" can be "night", except then the spelling cannot skimp on the space.