Determination of Shravana Vratam: Is it possible for the day of occurrence of Shravana nakshatram and the day of fasting for Shravanam to be different? Is it sufficient for the existence of Shravana nakshatram for 12 nazhigai-s from sunrise? If not, can we observe the fasting the previous day?

Shravana vratams are two kinds – some observe Shravana vratam to satiate Oppiliappan and others follow the methods followed by Oppiliappan sannidhi; as per their practice, Shravana vratam is undertaken on the day when the Shravana nakshatram exists for 12 nazhigai-s from sunrise.
If it does not (for example: Shravana nakshatram exists only for 11½ nazhigai-s from sunrise), they undertake the fasting the previous day.
Certain Panchangams indicate the Shravana vratam is in the evening. So, follow the practices of one’s own tradition. Many in our tradition follow the practices of Oppliappan sannidhi.