Błędnie: I was looking forward to the visitors' arrival on Monday, so I changed my plans completyely, supposing I'd be very busy at the beginning of the week. Poprawnie: was expecting the visitors' arrival on Monday, so I changed my plans completyely, supposing I'd be very busy at the beginning of the week.
"Look forward to" oznacza 'cieszyc się na coś', 'nie móc sie doczekać', 'oczekiwac z przyjemnością'. "Expect" to "oczekiwać", "spodziewać się".