Sri Krishna Jayanthi (also called Janmashtami, Sree Jayanthi, Gokulashtami, Krishnashtami), for some reason, seems to have split into several factions. Krishna was born on Rohini star. The day he was born was also an Ashtami (8 days into Krishna Paksha - the dark half of the moon). Today (Saturday) is not Rohini. Today, however, is Ashtami. Ashtami begins from midday and extends well into midday Sunday. Smarthas celebrate it on Ashtami - i.e today. Rohini begins Sunday early morning and extends all the way to Monday afternoon. So, some SriVaishnavas celebrate it when Ashtami & Rohini intersect (tomorrow). Some SriVaishnavas ignore Ashtami and celebrate it on pure Rohini star date (Monday).
Yadhaa yadhaa he dharmasya glanir bhavathi Bharatha;
abhyuktaanamadarmasya tadaatmaanam srijaamyahamh
- Srimad Bhagavadh Geetha: Chapter 4 Verse 7 on Krishna's Birth.