Dear Paran,
For us (Iyers), vittam kazhinjaettam naal gokulashtami. It means we are much concerned about the thithi. Iyengars follow Avani rohini. That actually comes this year, one month later. We celebrated avani avittam itself in AADI month this year. During, Agnatha vasa (mahabaratha), duryodhana found dharma a day before itself. But, dharma will say to him better you consult bhishma regarding this. He will be clarified then about circles of time. It is like that things will hapen sometime (this year everything has been changed). Even under normal circumstances, Gokulashtami (that is ashtami) and Shri jayanthi or pancharathra jayanthi (rohini) will mostly wont coincide. Might be they will come in succeeding days. Similar for Karthigai deepam also.