In general, trekking to Mount Rinjani is open from early April until the first or second week in January. This is in accordance with the Indonesia seasons where from April to October is a summer and October to early of January is moderate rainy season. The peak of the rainy season is usually between January to March which usually Mount Rinjani trekking activities will be limited.
It is advisable to book hotel rooms in the village of Senaru or Sembalun corresponding ascent path to be traversed, especially if you are going to climb mount rinjani on a high season between July to mid-September.
April and May is the quietest and you will experienced the most beautiful views since the entire savanna and trees are still green. The extra caution should be taken on the rainy seasons which starts on October, specially for trekking routes from crater rim Sembalun to the lake. In some places from lake to Senaru village , the walking route is slippery and in some places frequent landslides. If you decide to go on that rainy season, do not forget to bring a rain coat for trekking Mount Rinjani.