The given day must not be associated with Dasami for 4 Nazhigai-s before sunrise; that is, if on the previous day, Dasami exists for more than 56 Nazhigai-s, Ekadasi vratam is not observed the following day.
These calculations are based on the detailed study of Dharma Shastra, statements from Smruti, etc. and hence these differences are based on the traditions.