December has become the unofficial sigh in the increasingly frenetic, relentless TV calendar — a time for networks to lick their wounds before trying, trying again in January.

Barring a few squirrelly-looking cable reality shows, holiday specials and college bowl games — there will be dozens of those — the Christmas month represents a brief respite (for critics, especially) before the deluge of premieres scheduled for January.

Once, it was all so simple, if rather arbitrary: The TV season began in mid-September to coincide with car companies launching their fall models, then ended in April. Eventually, that was expanded through most of May, reflecting the importance of that month’s rating sweeps. Reruns followed. Repeat.

Now, the calendar is punctuated throughout the year by various trends and signposts. Premieres dribble out constantly, giving way to the periodic unruly spray. The volume is such that there’s ostensibly little rhyme or reason to it.

Look more closely, though, and some old rules still apply, while newer ones have taken hold. Here, then, is a 2011 planner to track TV’s crowded daze — with apologies to a certain Carole King song — through the winter, spring, summer and fall.

Winter

Beyond the aforementioned new-series invasion, the awards season has produced a glut of relatively new kudocasts in January, along with college bowls that now spill well past New Year’s Day and NFL playoffs scheduled in primetime.

The biggest sporting event of all, “American Idol,” also begins, officially bifurcating the old broadcast TV season. Other contests, the Oscars and Super Bowl, moved backward and forward, respectively, into February, where the sweeps no longer effect the month as they once did.

Spring

March and April trigger second waves of midseason shows — including the spring edition of “Dancing With the Stars” — as well as the NCAA basketball tournament and customary cable glut.

Between the upfronts, sweeps and season finales, May remains frenzied, albeit with conspicuous changes. There’s no longer the same influx of sweeps-related miniseries and events, but networks still aggressively pursue the cliffhanging ritual on episodic shows.

While TV movies have largely disappeared from the broadcast menu — lavish two-parters were once a May fixture — you can count on HBO to roll out a couple of prestigious movies as awards bait, capitalizing on the month’s proximity to the fast-closing Emmy-eligibility window.

Summer

June marks the start of the summer calendar, with classy cable dramas and a medley of broadcast reality shows rushing to fill the void left by the networks’ scripted series. Nothing fits the latter job of summer time-killer more symbolically than CBS’ “Big Brother,” which airs several times a week, as well having an “After Dark” iteration on Showtime.

A recent twist, meanwhile, includes testing Canadian dramas during this stretch, so when the weather warms, it’s not unusual to hear an actor say something like, “Aboot what time did he leave the crime scene, eh?”

Cable premieres extend into July and August, allowing a returning show like “Mad Men” to conduct its latest season while staying fresh in the mind of Emmy voters.

Fall

The major networks have come back around to a September “premiere week” that accounts for the lion’s share of their new fall offerings. But cable, notably, no longer fears competing for attention with broadcasters as it once did.

Instead, channels like HBO and Showtime have launched key series like “Boardwalk Empire” and “Dexter” directly into the teeth of the network crush, just as AMC introduced its hit “The Walking Dead” in October — feeling no cause to fret about November sweeps competition.

Strong ratings for pro football, of course, had a huge impact this fall, but labor discord between owners and players threatens to disrupt that scenario next year, for both the NFL and NBA.

Regardless, the networks can rely on two things happening each October: Baseball’s World Series, and the customary panic that ensues when newcomers flop more spectacularly than ever (see Fox’s “Lone Star”).

Oh, and November 2011 will signal one more watermark for TV news: One year and counting to the 2012 presidential election.

By then, to gather strength for the barrage to come, everyone will need another laid-back, drama-free December. And remember whatever the month, when it comes to TV, you’ve got a friend.

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