"We visit our sister in July."
Translation:W lipcu odwiedzamy naszą siostrę.
I can say "I am visiting sisters tomorrow" (in which case I am probably speaking quite informally, or even "I am sister-visiting", which is even more informal to the point of being jocular) but the sentence specifies "our", so does need "naszą" for the translation to be correct.
The basic difference is that in English you need either an article or some other determiner like for example possessive pronoun. While in Polish it is not necessary.
For example, normally, you wouldn't say in Polish "Dziewczynka uderzyła swoją głową o stół" but rather "Dziewczynka uderzyła głową o stół". While in English you would say "The/a girl hit her head against the/a table". I put aside the fact that "her" could mean that she hit a head of some other woman and not her own head.
We were told to mark all correct translations for "We visit our sister in July". I marked only ' W lipcu odwiedzamy naszą siostrę' as correct, but they say I should have also marked the option in which 'naszą' was omitted. In view of the comments below, can you please confirm whether or not I can omit 'naszą' in this sentence? Thanks.
Well, for the same reason that basically every noun changes - the grammatical cases.
"lipiec" is Nominative, the basic case, the one you'd find in a dictionary, used mostly for the subject of the sentence. But to say "in [name of a month]", you need the preposition "w" and the Locative form of the name of the month (or basically any other noun, when you want to say "in/inside [noun]". The Locative of "lipiec" is "lipcu".