Piccam Ekadasi means the remaining Ekadasi (tail end of Ekadasi) on the subsequent day and vratam has to be observed on that day as well.
The first quarter of Dvadasi tithi is known as Harivasaram; first, we must calculate for how many nazhigai-s Dvadasi is present; for example, if Dvadasi is present for 60 nazhigai-s, then the first 15 nazhigai-s is Harivasaram which is a balance of Ekadasi during which fast must be observed.
Sometimes, even when Dvadasi starts the previous day itself, the end of the first quarter of the subsequent day would be Harivasaram; depending upon the nazhigai-s of Dvadasi period, the first quarter varies; Harivasaram period is indicated in the panchangam.