If my father has a day off, we always go to see my grandfather.
The sentence expresses a habitual situation in the present
We can express this situation in the past as follows:
If my father had a day off, we always went to see my grandfather.
But it will be more appropriate if you replace if with when.
If my father had a day off, we would always go to see my grandfather.
It's usually considered a hypothetical sentence expressing an unreal situation in the present; there's no question about it. I think to make it a habitual situation in a clear way, you use when instead of if as follows:
When my father had a day off, we would always go to see my grandfather.