Kedarnath — The Abode of Shiva

Kedarnath is located in the Himalayas in the state of Uttarakhand. It is 11,755 ft above sea level and closes to the source of the Mandakini river. Doors of the temple open on Akshaya Tritiya (usually falls at the end of April) and are closed soon after Kartik Purnima (late October or early November) for a duration of about 6 months. So if you want to visit Kedarnath that is the only time when you can. (Kedarnath Tourism)
But let's get to practicalities. Tourists & devotees can travel Kedarnath in bed weather but it’s not safe & recommended to travel in Monsoon season & on a rainy day. Hills around will remain snow-capped throughout the year. Though the weather would be pleasant during the day, if it rains, the temperature during day time can drop to near Zero degrees Celsius even in the month of May. And when it rains staying there will be very difficult if you have not got proper clothing. My, advise — either visit in May or the first half of June or visit after monsoon is over.
Currently, the government is allowing only 5000 pilgrims in a day for which you need to do the registration. There are two ways to get registered for Kedarnath yatra (along with for Char Dham yatra). You can either do registration directly at the registration counters or through online registration. Government has made counters at public places like railway stations, market places, near the bus stand in Haridwar and also at the Chardham Yatra terminal in Rishikesh. This terminal in Rishikesh is located at the compound of ISBT.
Kedarnath is a steep 16 km trek from Gauri Kund (ponies and palanquins are available for hire). There are also about eleven aviation companies that operate a helicopter service to Kedarnath from Gupt Kashi, Phata, and Sersi.
The government approves licenses for aviation companies every year and so usually the helicopter tickets become available for booking only by end of April or the first week of May.
There are many hotels and guest houses in Phata as well as Gupt Kashi, but it's better to book hotel rooms beforehand. For detail, information check follow this — Kedarnath Tourism