Assateague Island is almost uninhabitable in summer, which is fine because that's when everybody goes there. From September through May the 37-mile strip of surf and sand and sighing forest is left largely to lovers of great birds, small beasts and gritty solitude.

Fall is fine and bright or fine and fierce there, and winter as wintry as any might ask.Early spring is invigorating but can be punishing, for the ocean that tempers the decline of the year resists its reawakening. April is the cruelest month.

And then there is May.

May on Assateague - and June before the public schools belch forth - is a subtle shifting of tones from grayish brown to brownish green and a riot of courting and nesting birds. There is one magic period of a few days to a week when the days are almost hot and the nights are almost warm and the mosquitos and greenhead flies are almost absent.

The bset way to do Assateague is to camp, for if you are there only in daylight you miss the things that go snurf in the night. Most of them are those famous, ugly and dull "wild" ponies, but some are the tiny sika deer, full-grown to the size of a whitetail fawn. The incredibly loud alarm whistle of a surprised sika shocks and stuns like a whip.

In the summer Assateague often is so crowded there literally is not one parking place to be had on either the Maryland or the Virginia section of the island. Some 10,000 would-be cameras are turned away from mid-June to September.

All of which is by way of saying go now. There is ample space in the public and private campgrounds on both states' sections of the island, ranging from tent-only sites to concrete pads with full hookups for recreation vehicles, with prices from $2.50 to $7.50 a night.

Better yet in terms of experiencing the island are the three hike-in and three canoe-in camps, but their combined capacity is 120 people, so it would be wise to telephone ahead, at least on weekends. Stays are limited to two days at the hike-in sites (two of which are one hell of a hike on the sand) and three days at the canoe camps (Tingles Island, Jims Gut and Popes Bay). The national park use fee is $1 a day, but they usually don't start collecting it until mid-June.

The Maryland end is approached via Route 376 from Berlin, Md. The state park office number is 301/641-2120; the National Park Service number is 301/641-1441.

The less-developed Virginia end is approached from the Town of Chincoteague, which is not yet entirely succumbed to tourism but is falling fast. The park information number is 804/336-6577.

It should be said that anyone whose main interest is swimming or sunbathing might as well forget spring on Assateague.Wind and water are cold, and the sun (because at this season it strikes the sand most directly) will fry a winter-pale hide while you shiver. Bird lovers need not be told how rich the island experience is, but canoeists and backpackers prepared to invest the necessary energy will be overwhelmed by the other wildlife, from sand crabs to river otters. In parts of the forest it is possible to see the island much as Captain John Smith did.

Take a sturdy tent, because the back country campsites are exposed to winds that frequently gust over 50 knots, and more clothing than you think you'll need. Water must be packed in, and take plenty. TO GET THERE

Both ends of the island are approached via U.S. 50 to Salisbury, Md. For the Maryland section, continue on 50 to U.S. 113; turn right to Berlin and left on Route 376 to the island. For the Virginia section, take U.S. 13 south from Salisbury to Route 175 to Chincoteague. At the traffic light turn left and then right at the Assateague sign. CAMPING INFORMATION

MARYLAND

Assateague State Park: 311 sites that can accomodate any type of camping unit. It's about a hundred yards from the beach with no ocean view. Central hot showers and flush-toilet bathhouses. $5 per night, limit of eight persons to a site, 14-day limit (sevens days during summer). No pets. For information: 301/641-2120.

Assateague Island National Seashore: two primitive campgrounds (closed in winter) plus two hike-in camps and three canoe camps. North Beach has 86 units 100 yards from the beach, Bayside 40 units, 400 yards from beach. No ocean view. Drinking water, chemical toilets, picnic tables. Fee $2.50 per night from mid-June to Labor Day; visit limit 14 days (seven days from Memorial through Labor days); six persons to a site. Pets must be leashed at all times. The hike-in and canoe campgrounds are open year-round and have a capacity of 20 persons each. There are chemical toilets and picnic tables, no water; fee $1 per night during summer, limit of stay two days at hike-in sites and three at canoe camps. No pets. For information: 301/641-1441.

There are several commercial campgrounds in the Berlin area, some with full hookups, that charge from $2.50 to $7.50 per night. VIRGINIA

The only public campsite on the Virginia end is a hike-in with a capacity of 20 persons and a two-day limit. It's a 3 1/2-mile hike from the visitor center, where parking is available. All food and equipment must be packed in. Pets are not permitted anywhere on the Virignia end of the island. The campground is open year-round, with $1 daily use fee collected during summer only.For information: 804/336-6577.

There are a number of commercial campgrounds on Chincoteague Island and its mainland approaches, with fees varying according to the type of hookup (if any).