It is commonly used as two words. When I tried it as one word, both my spell check and the spell check in the top right corner of this answer box marked it as misspelled.
The word 'field trip' is considered an open or spaced compound noun.
'Field trip' is two words, not one.
Two words - "one day".
Just one word.
The best description is forethought and planning.
You can rent a car from Enterprise.com. They will allow you to rent the vehicle for a one day period. You will need to set up two different rentals for this occasion as you will need one for the trip and one for the return of the trip.
'Good Day' is two words.
Day shift is two words
Race day is two words.
Bus station is two words. My mother took me to the Greyhound bus station for my trip to Arizona.
It is properly written ALL DAY - two words.
Someone is one word when you are referring to a person.