Dewa Sanzan means "Three Mountains of Dewa" and indeed comprises the three sacred mountains of Mt. Haguro (羽黒山), Mt. Gassan (月山) and Mt. Yudono (湯殿山), clustered together in the ancient province of Dewa (modern-day Yamagata prefecture). Holy to the Japanese Shintō religion and especially the mountain ascetic cult of Shugendō, Dewa Sanzan is a popular pilgrimage site visited by many, including famed haiku poet Matsuo Bashō on his Narrow Road to the Deep North. If you're lucky, you may even spot a yamabushi ascetic pilgrim blowing into a conch shell.
When to go
Mt. Haguro is accessible all year round, but Mt. Gassan and Mt. Yudono close during winter because of snow. The (short) peak climbing season lasts from July to mid-October. Festivals are held to celebrate the opening and closing of the shrines.
Regular buses from Tsuruoka (~40 min; ¥800) connect to Haguro-machi (羽黒町). The buses depart from the front of Tsuruoka Station -- there are other lines, so make sure to board the right bus. For the Mt. Haguro trail head, get off at the "Haguro Center" (羽黒センター) stop. The bus continues another 10 minutes to the summit of Mt. Haguro (羽黒山頂). For Tsuruoka bus schedules, see the Shoku bus website (Japanese). After 15 minutes, the bus continues to Gassan Hachigōme (月山八合目), the trail head for Mt. Gassan. Check the bus schedule carefully -- there are special schedules during holidays. On the bus going from Gassan-Hachigōme back to Tsuruoka, change buses at the summit of Mt. Haguro. Occasional buses connect Yudono-san to Yamagata.
In Tsuruoka Station, the information desk can provide some basic information and a map of the Dewa Sanzan area, both in English. However, there are few other sources of information in English -- try to come with a Japanese speaker.
Buses shuttle from both the top of Mt. Haguro and the Kyuka-mura stop at the base of the mountain to Gassan Eighth Station. Note that the bus stop is on the other side of the mountain (not in Haguro-machi), take the footpath down (40 min) instead of the road to get to the stop faster.
See & Do
The traditional approach is to hike across all three mountains, although most visitors opt for a bus from Mt. Haguro to Mt. Gassan as the distance is quite long. One can leisurely climb up Mt. Gassan and come down via the Mt. Yudono in one day.
Hagurosan is the most easily accessible of the three sites and the only one that can be visited without some hiking.
Mt. Gassan and Mt. Yudono
The hike to the top of Mt. Gassan and across to Mt. Yudono is a relaxed full-day hike, thought it should be approached with due respect. Obtain a map of the area from any nearby tourist office before you set out and take heed of weather conditions. With a forecast of rain and without proper rain gear, hiking to the peak can be not only extremely uncomfortable (you will get soaked) but dangerous as well with high speed wind gusts, slippery rocks, and very cool temperatures near the top.
You may want to stop by bus at Ōami and visit the Dainichibō Temple (大日坊) or the Chūrenji Temple (注連寺). To get there, take the Mt. Yudono leg bus and get off at Ōami. Unfortunately, bus schedules may not mesh well with various interesting temple tours.
Buy, Eat & Drink
Bring plenty of water for Mt. Gassan and Mt. Yudono, as there is little to be had along the trail. There are places to eat at the 8th and the 9th station and on the top of top of Mt. Gassan. Accommodation is also available at the 8th station and in a mountain hut near the Mt. Gassan summit; reserve in advance if you want to be assured a place for the night.
There is a small shopping center near the Mt. Yudono bus stop below the temple, which has a number of restaurants and many souvenir shops.
Most pilgrims opt to stay in the town of Haguro-machi, at the foot of the mountain. There are over 30 shukubo here offering basic lodgings. Regardless of where you stay, though, you must arrive before the dinner hour or you will have a difficult time finding a room. The best way is to call in advance and make a reservation (in Japanese). The shokubo experience is a good way to enjoy the journey, provided someone in your party speaks Japanese.