"They wait for midnight."
Translation:Aspettano la mezzanotte.
If you include the definite article before a day of the week, for example lunedì, you are changing its meaning from Monday (lunedì) to Mondays (il lunedì). For example, "Vanno al negozio lunedì" means "They are going to the store Monday". (This is an example of the present indicative tense combined with a specific time reference to convey a future action.) Conversely, "Il lunedì, vanno al negozio" means "Mondays they go to the store". In this usage, with the article, it conveys an event that occurs regularly on the specified day of the week.
no. 'lunedi' and 'il lunedi' as used by LINBUR0100 are adverbs (adverbial phrases). they can translate to 'on monday' and 'on mondays' respectively. the above sentence is referencing the time of midnight, a noun. to use it as an adverb you would need a preposition before it. 'by midnight'--'entro mezzanotte', 'after midnight'--'dopo mezzanotte', 'until midnight'--'fino alla mezzanotte'.
this has been answered several times already on this page. the meaning of some italian verbs incorporates a preposition as compared to the english equivalent which does not. aspettare is one of them. it can mean 'to wait for', or if you prefer; 'to await'. as 'aspettarsi' it means 'to expect, to anticipate, to foresee' 'essere' does not include the meaning of prepositions within.