YouTube's CPM (or "earnings per 1000 views") fluctuates on a daily basis. Larger fluctuations occur during different times of the year. For instance the end of the year will always have the highest CPM because of the holiday advertising. January (right after the holidays) and July (in addition to other summer months) typically have the lowest CPM's.
CPM also fluctuates based on what country in which the ads are viewed, where ads are viewed (mobile and external website views have lower-value or no ads) and the demographics of the channel.