Allergan disclosed the special meeting date in a court filing in Delaware. Officials with both companies declined to comment.
Hedge fund Pershing Square Capital Management, which has partnered with Valeant in the takeover bid, sought the special meeting to elect six new members to Allergan's board and force the deal through.
Valeant has offered about $53 billion in cash and stock for Allergan, but the Orange County company has rejected the offer as too low.
The proposed deal would give $72 in cash and 0.83 shares of Valeant stock for each share of Allergan's stock.
Valeant has argued that the acquisition would benefit Allergan shareholders because it would make the company more profitable by reducing Allergan's research and tax expenses.
Allergan initially said Valeant's plan to trim its research budget was short-sighted, but later announced its own plan to cut jobs and increase profits.
The takeover attempt has been one of the fiercest on Wall Street, with both sides filing lawsuits and accusing the other of distorting facts.
On Friday, Valeant and Pershing Square delivered to Allergan requests from more than 30% of shareholders to hold the special meeting.
Allergan said in a court filing that it scheduled the Dec. 18 meeting without reviewing the validity of the requests. Its board scheduled a Sept. 4 meeting to determine whether the requests for the meeting are valid.
Allergan shares gained 56 cents, or 0.3%, to reach $163.97 on Tuesday.