Fasting begins from the first light of the day and this occurs in the early hours of the morning. To make it easier, many mosques & Islamic Centres put together timetables which have the starting times. There may be some differences in these timetables based upon the method they used to extract the times.
Traditionally, most Muslims follow the observations made by Muslim World League or University of Islamic Sciences, Karachi who have agreed with classical scholars such as al-Biruni that we the correct times are based on about 18 degrees.
In the UK, we suffer from persistent twilight in summer months for which a new standard was established in 2012 by London Central Mosque and have the starting time suhoor later.
Those who wish to be cautious can follow the earlier times, and these are reflected in our Fajr Staring times. Our Ramadan calendar follows the times published by ICCUK.
A more full and complete explanation can be found at moonsighting.co.uk.