Please accept my humble obeisances. All glories to Srila Prabhupada!
Each year the BBT in North America publishes a Vaisnava calendar containing the dates of Ekadasis, appearance anniversaries of Lord Krsna and His expansions and incarnations, appearances and disappearances of our acaryas, and more. Because of the way the calendar is calculated, based on Vedic astronomy, the dates occasionally differ by one day according to your geographical location. The dates are calculated for Los Angeles, with alternate dates (if applicable) for Chicago, Toronto, and New York City. Generally, if you live in or near one of those cities, or roughly north or south of one of them, you can be confident of the date given on the BBT calendar.
But the imminent Ekadasi coming up on the 10th or 11th in North America is an exception. In this case, devotees in Los Angeles and all devotees living in places due north or south of LA, or west of the city (including in Hawaii), should observe it as stated on the calendar, namely on the 10th. But those devotees living even a bit east of LA, including in San Diego, Boise, and, believe it or not, Laguna Beach, should observe Ekadasi on the 11th.
To be absolutely certain of the Vaisnava dates in your area, please visit http://www.krishnadays.com/ and downloaded the free Gcal program.
Your servant, Dravida dasa