FORGET about the calendar. At Lincoln Center it’s already midsummer!

At the Lincoln Center Plaza the joint is jivin’ and jumpin’ to the big-band sounds of “Midsummer Night Swing,” that now traditional alfresco pop dance marathon, which has just opened and runs for a couple more weeks.

Meanwhile, in a more staid fashion, at the adjoining New York State Theater, the New York City Ballet is finishing up its spring season, equally traditionally, with a week of George Balanchine’s “A Midsummer Night’s Dream,” to the big-band sound of Mendelssohn.

It is a strange full-evening ballet, because, although it is in two acts, Shakespeare’s story is limited to the first act, with the second a virtually plotless, pure dance divertissement.

The first part of the ballet – the bit about the fairies in the woods – is perhaps inferior, and certainly far less Shakespearean in spirit, than Frederick Ashton’s balletic version of the story. Still, it’s a sound enough work and offers a pleasant way to while away a summer evening.

After this long, grueling season marking City Ballet’s 50th anniversary, during which it has staged 100 different ballets, the company finally seems to be showing a little strain and looking a tad tired. No wonder.

The two principals, Wendy Whelan as Titania and Damian Woetzel as Oberon, looked end-of-the-seasonish, but Tom Gold (who is to make his own debut as Oberon this weekend) made a sprightly and funny Puck.

Jenifer Ringer charmed as a more than usually desperate Helena, while Miranda Weese, staunchly partnered by Nilas Martins, led the second act revels with smooth finesse.