When you have specific dates in the past, do you always have to use the simple past?
On 01/31/2013 I met my friend. We were/have been together all day long.
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You have to use the simple past when you are referring to actions that happened in the past and are no longer in place.
So you have to say: on January 31st 2013 I met my friend. We spent [the time] (were) together all day long.
You may use the past perfect if you want to talk about something that happened while you were together:
January 31st 2013 I met my friend. We had been together for more than an hour when we ran into an old friend.
Past Tense refers to something in the past, with no connection to the time now. So if you have a date in the past time you use Pt (past tense).
As a day is not a point in time (most but not all lasting 24 hours), both punctive and durative events can take place on any given day.
On the Second of May, I met my friends at the lake. We were fishing when suddenly a shot rang out in the woods. [simple past / past continuous / simple past (MWV)]
It is more natural to use simple past tense here as at the start of the sentence you have already set the time reference to the past and I see no reason not using were in we were together all day long.