The admissions deadline dates depend on the course commencement dates. Most b-schools have more than one round of application deadlines.
For courses that commence in Aug/Sep, b-schools usually have Round 1 deadline in September of previous year, Round 2 deadline in December of previous year, and Round 3 deadline in March of the same year
For courses that commence in January, b-schools have Round 1 deadlines in February, Round 2 deadlines in April, Round 3 in June etc.
Many b-schools have rolling admissions until a particular last date, so you can apply anytime before that date.