After visiting my father in March, he's sinced developed Alzheimer's quite seriously. My older sister told me (at the end of April) that he (my father) didn't know that she had two younger brothers.
Neither my brother nor I knew how to deal with this (contacting him might have been, at best, confusing for him, or, at worst, distressing) so we haven't spoken to him since our sister mentioned his condition.
However, today being Fathers' Day, I phoned him on the off-chance, and fortunately he was mentally coherent and recognised me. He was very happy to have had a Father's day greeting over the phone, and I was pleased that he'd returned (albeit, most likely, only temporarily) to 'normal'.
I'm visiting him in three weeks time, hopefully coordinating a visit from both my brother and sister and their respective children, as well.
He was 'thrilled' to hear from me, which also made my day.