Yes, it's correct. In fact, it is from classical English (Shakespeare). Here you see the subject relative pronoun missing. (There isn't a single day (that) goes by.......) This was perfectly OK in old English, and was often used by Shakespeare. Nowadays, common phrases using this structure are rare.

There are actually 3 errors in this sentence. 1 very obvious mistake, 1 mistake made by many native speakers, and 1 mistake that isn't a grammatical mistake. 1: The obvious mistake: you're starting a relative clause without a relative pronoun: "...a single day goes by..." should be "...a single day THAT goes by..." 2: The mistaken usage of the relative pronoun "that" in the part of the sentence that says: "that I don't think of it." Firstly let's simplify this sentence, keeping the mistake in place, by taking away the first relative clause ,which isn't necessary, so we're not distracted and the error becomes much more obvious: "There is not a day that I don't think about it." This should be: "There is not a day when I don't think about it." This is an error many native English speakers make. Some examples: "The day the music died" > "The day when music died" and NOT "the day that music died" "The day I saw you" > "The day when I saw you" and NOT "the day that I saw you" note: this mistake is so overwhelmingly common that some people might argue it's not a mistake if so many natvie speakers say it this way.. it's still grammatically incorrect though :/ Use one of these: "There is not a day that goes by when I don't think about it." "There is not a day that goes by without me thinking about it." 3: Say "think about it" instead of "think of it" "Thinking about" implicates extensive thought about something. "Thinking of" implicates an inextensive thought OR an opinion of something. ex: - I was thinking about how to get there. (extensive thinking/planning a route) - I thought of a pretty girl when I had to kiss that frog. (inextensive thought) - I always thought of you as a brother. (opinion) - I think of you as being rude. (opinion) Regards, Sertan

Close. Here's how it should be: "There's not a single day that goes by that I don't think of/about it.

It is perfectly fine.

it doesn't sound correct , we can simply say : " I think of it evey single day "