I am confused whether I should use "didn't" or "don't" in the following sentence.
There has not been a day when I didn't/don't think about you.
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If it's all in the past, for example when somebody was away but now they are back (possibly today, possibly some days ago), you use simple past for both verbs:
There was not a day when I didn't think about you.
If the person has returned today, the situation has just ended so you could also use present perfect for the first verb and simple past for the second verb
There has not been a day when I didn't think about you.
If the person is still away, this situation is still true so you would use present perfect and simple present
There has not been a day when I don't think about you.