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@Chetan Kulkarni ji...Day 6: Gangtok- Tsomgo- NathuLa-Gangtok
Day 7: Gangtok to Lachen
Day 8: Lachen- Gurugongmar- Lachung
Day 9: Lachung-Yumthang-Yumisangdong-Gangtok
If day 6 is a Monday then you will not be allowed to visit Nathula as Monday is for army vehicle movement.
Day 7 you can go to Thangu from Gangtok (after taking necessary ILP ) so that on Day 8 you can visit Tso Lhamo Lake along with Gurudongmar Lake and by evening reach to Lachung.
And on Day 9 add Khatao in itinerary and do start very early on Day 9 for Yumthang and Zeropoint.
Just for information... Tso Lhamo Lake is highest Lake of India at 17400 feet and is just 8 km from Gurudongmar Lake. Gurudongmar Lake is second highest lake of India at 17100 feet and it is 15th Highest of World.
How to visit Tso Lhamo Lake ?
ALL THE BEST