The IRS announced that they will begin accepting 2013 business tax return on Monday, January 13, 2014. This includes corporate tax returns (Forms 1120 and 1120S), partnership tax returns (Form 1065), trust and fiduciary tax returns (Form 1041), excise tax returns (Form 720), and payroll tax returns (Forms 940 and 941). However, unincorporated businesses (those filing a Schedule C, E, or F on their Form 1040) must wait until personal tax return filing begins on January 31st.
The IRS announcement is here.