Most IITs have a month long vacation in December i.e the winter break after the first semester which also marks the beginning of the placement season.
Generally, the high paying firms such as banks, consultancies and those providing jobs in coding profile are invited in the initial days of placements. Most students are attracted towards such jobs and tend to prefer them over the core technical ones which do not generally pay as high as the non core ones.
In order to offer students with a much more balanced choice, some IITs tend to provide the core technical firms some slots in the first few days even though they don’t offer high paying jobs. IIT B did the same last year after MHRDA requested IITs to do this in order to support Central Government’s ‘Make In India Initiative’. (This placement season, IIT Bombay to push core engineering companies to Day 1 slot – The Economic Times)
Most IITs tend to wrap up their placement season by March which typically means a 3–4 month long placement season. There are a lot of market factors which come into play too (e.g recession in 2008 led to an extended placement season). Starting in December gives most IITs ample time to achieve desired results in placements.