I can third this. Usually companies are busy making plans for the new year. Then somewhere near February is where they start looking for the people who can make it happen.
Big companies usually operate on a yearly / quarterly cycle when budgeting things (bookkeeping cycle). If they need someone in January you're probably hired in December. January is the new start of the year. If they decide to take on new people it takes a few weeks before all the meetings are done and those positions become available on the market. Then summer is usually a bit more quiet as well.