Eat, drink and be merry; in a month you may not be old enough.

Gov. Edward J. King Thursday signed a bill raising the legal drinking age in Massachusetts from 18 to 20 and said he plans to enforce the law next month, even though the state legislature this week removed the emergency preamble putting the law into effect April 1.

King, who had lobbied for the drinking age to go up to 21 but settled for 20, wants the bill to go into effect before spring fever hits college campuses and before proms and graduations hit high schools. By filing a letter with the Massachusetts Secretary of State, he can enforce the law at any point within the next 90 days, the normal interval between signing and enforcing a law.

State Rep. Robert B. Ambler, who helped draft the law, suggested April 9 as the date the law should take effect because it is "the beginning of the religious season." That date would allow schools and public establishments enough time to inform teenagers of the new law and any resulting policy changes.

Several other legislators with large teenage constituencies have warned that raising the drinking age at midnight on April 1 would encourage last-minute parties, especially since March 31 is a Saturday. State Rep. William G. Robinson said if the measure had taken effect on April Fool's Day, "the legislature and governor would have been a damn laughing stock." He recommended allowing the usual 90-day grace period because students would be out of school when the law goes into effect and therefore less likely to protest.

A joint conference committee of the state House of Representatives and Senate formulated a compromise measure raising the drinking age to 20 in a single afternoon. The compromise followed the House's vote to phase in a drinking age of 21 and the Senate's decision to raise the drinking age to 19.