Unless there's been a major calendar change, the barge procession is a 7-8 day event, not a monthlong event (Example 2013). It's held at the end of the Buddhist lent, the week preceding the October full moon.
The Telegraph appears to have gotten their information from the Tourism Thailand website which also list the event as a monthlong event.
Of course Tourism Thailand has a disclaimer which reads in part "... information may change without notice ..."
