The Senate vacancy created yesterday by the resignation of Harrison A. (Pete) Williams Jr. (D-N.J.) will be filled in a matter of days, or perhaps weeks, or maybe months.

The Garden State's newly elected governor, Republican Thomas H. Kean, isn't sure. He said yesterday in Trenton that he hadn't given the matter "the kind of deep and important consideration that an appointment of this kind deserves," and wasn't sure when he would get around to it.

The open seat presents Kean's two-month old administration with some delicate political choices at a time when New Jersey is in a state of upheaval over congressional redistricting.

A plan devised by the Democratic-controlled legislature was thrown out last week by a panel of three (Republican-appointed) federal judges. Outraged Democrats are appealing to the Supreme Court and threatening to postpone the June 8 primary until September.

Meanwhile, Williams' seat comes up for grabs this year, and the battle to fill it already is in full cry. On the Republican side, which has seen most of the action so far, Rep. Millicent Fenwick (Lacey Davenport to readers of the comic strip "Doonesbury") and Jeffrey Bell, a former Reagan speechwriter who ran unsuccessfully for Senate in 1978, are announced candidates.

On the Democratic side, former representative Andrew Maguire is the most prominent candidate, but Rep. James J. Florio, who narrowly lost to Kean in last year's gubernatorial contest, is pondering whether to try another statewide run.

Kean has three options on the vacant Senate seat: appoint someone who is an active candidate, appoint a caretaker, or wait until after the Republican primary and appoint the winner.

Political sources say that if he chooses the first option, his appointment could be, not Fenwick or Bell, but Rep. James A. Courter, Kean's campaign chairman last year.

Courter made it known earlier this year that he would like the governor's endorsement for the Senate seat and, if it opened, the appointment as well.

But as the question of Williams' expulsion lingered for months in the Senate, the chances of Courter's getting into the Senate race grew more remote, political observers said.

With Fenwick and Bell both far out in front in terms of name recognition among GOP voters, and with their campaigns in full throttle, Courter has lost valuable time, these sources say. It is not clear if he could make up the necessary ground, especially if the June primary date holds.

Courter said yesterday he will announce next week whether he plans to run for the Senate. Most observers say his entry would hurt Bell, with whom he shares a basic conservative philosophy, more than Fenwick, who carries the moderate banner in the party.

The court decision revoking the Democratic plan on redistricting may have reduced Courter's interest in the statewide race, however. Under that plan, his district would be paired with that of Republican Rep. Marge Roukema.

GOP leaders say that the plan, which Gov. Brendan T. Byrne signed into law two hours before leaving office, was designed to turn what is now an 8-to-7 Democratic congressional delegation into one that could become 10-to-4 Democratic.

One district in the plan would span a stretch of the Atlantic and was dubbed the "Contiguous-at-Low-Tide-Only District" by infuriated Republicans.