I'm staying until the day after tomorrow.

staying noun

— A stay or visit.

until Preposition

— If something is true until mathx/math, it is true for some time before mathx/math and then is not true. We didn't have any snow until the end of December. He worked until he was 72 years old. Until now, there was nothing we could do.

the Determiner

— Used, instead of a, to reference something specific, already known to exist. Compare "I read a book." and "I read the book." — Used with a stress, to show that the word following is special. Are you the John Smith that I went to school with? — Used with an adjective that acts like a noun to mean all of the people concerned The poor are always with us. — Used with superlatives forms of adjectives and adverbs. You are the best.

day Noun

— A day is a measure of time. — 24 hours — Seven days make one week — The time between midnight and the following midnight (or between sunset and sunset in Jewish reckoning) — The time between sunrise and sunset, when it is daylight — The part of a day spent at work or school He spent two days at work means that on two days he went to work; he did not spend 48 hours at work.

after Preposition

— Later in time. He came home after Jane did. The doctor came after the patient had died.

tomorrow Pronoun

— Tomorrow is day after today. Tomorrow is my day off. I'll see you tomorrow evening. I will go to school tomorrow. Tomorrows meeting should be short. — Tomorrow is the future in general. The city of tomorrow will be much cleaner than cities now.