Could you tell me if there is any difference in meaning between the following sentences?
I wasn't at Julie's birthday, because I worked the whole day yesterday.
I wasn't at Julie's birthday, because I was working the whole day yesterday.
These both mean the same, generally, but would normally be used in response to different situations.
If someone asked about the total amount of time you were doing something:
How much do we owe you?
Well, I worked the whole day yesterday.
If someone wanted to confirm your location at a certain time:
Where were you yesterday afternoon?
Well, I was working the whole day yesterday.