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The start of Ramadan is determined by sighting the new moon.  When the new moon is not visible due to clouds, then thirty days must be counted for the month of Shaban before the start of Ramadan.

Abdullah ibn 'Umar, radiyallaahu 'anhumaa said, that Allah's Messenger (sallallahu 'alaihi wa sallam) said:

"Do not fast until you see the new moon and do not cease fasting until you see it, and if it is concealed by clouds then count out the days of of Sha'ban for it"
(al-Bukhari)

Abu Hurairah, radiyallaahu ''anhu, said, that Allah's Messenger (sallallahu 'alaihi wa sallam) said:

"Fast when it (the moon) is seen and cease fasting when it is seen, so if it is concealed by clouds, then complete thirty days of Sha'ban."
(al-Bukhari)

Trying to pre-empt Ramadan by fasting a few day early is forbidden.

Abu Hurairah, radiyallaahu 'anhu, said, that Allah's Messenger (sallallahu 'alaihi wa sallam) said:

"Do not pre-empt Ramadan by fasting a day or two before it, except for a man fasting his usual fast - then let him fast it"
(Muslim)

The end of Ramadan is also determined by sighting of the new moon.  When the moon cannot be seen, Muslims must fast for thirty days.

Adiyy ibn Haatim, radiyallaahu 'anhu, said, that Allah's Messenger (sallallahu 'alaihi wa sallam) said:

"When Ramadan comes to you then fast thirty (days) unless you see the new moon before that"
(at-Tahaawee)