You have to observe as per your panchangam. As per drik panchangam you have to observe on 25-11-20. As per paampu vaakkiya panchangam you have to observe on 26-11-20. If during sunrise time , ekadasi thithi is there you have to observe ekadasi on that day.
for viratham purpose sun rise time should be taken. for annual ceremony purpose you have to take sun set time should be taken.
