Several periods are to be noticed:
New Year: the release of Q4 result. For financial, high-end consumer goods, construction (and related machinery), transportation, raw materials, automobiles, and oil industry, ask for God's mercy.
Chinese New Year: lay-off wave.
March and April 2009: release of 2008 annual result. Again, for the above mentioned industries, unless for the highly conservative corporates, otherwise, again, pray.