Most of the festivals in Nepal are based on position of the moon i.e. Tithi. A lunar day or Tithi is the duration taken by moon to travel 12 degrees on its orbit respective from the new moonThe Bikram Samat calendar we use today is just a solar calendar with 365 (366 on leap year) guided and in sync with Luni solar Bikram Sambat calendar calculated through Panchang Tithis. Please note while Panchang tithis and Nepal Sambat tithis are similiar in concepts, both being based on Lunar calender, there are differences. While Panchang tithi strictly follow rotation of moon and can have overlapping tithis (in a solar day), the NS takes tithi at the time sunrise as reference for the entire day. Whichever tithi the moon is at the time of sunrise, the entire day is named after it. 
This calendar show the distribution of festivals as per the Lunar days (tithi), fortnight (paksa)  and months (mas). The Nepal Sambat Months has been used as a reference
This is supposed to be collaborative effort, if you are interested to add jatras and other festivals to this calendar, please contact here, your contribution is highly welcome. I expect this to contain errors, as I am not an full expert on this. Any correction can be suggested here. Regards.
A lunar month i.e time from one new moon to another, is on average 29.5 days (this alternates as 29 and 30 days in NS calender). As a result the year has 354 days not 365 days which is synchronized by adding intercalary month or extra month every 3 years called Manamas month. As you can see on above calendar the 30 tithis in each month totals 360 days in the year, based on moon position some lunar tithis are merged into one day which collectively cumulates  to 354 solar days.As you will realize, due to leap month (Manamas), some tithis may not fall on the Nepali months they are supposed to fall on, for example Fagu Purnima on 2080 falls on Chaitra. There are few festival which are not based on lunar position some of which are as followsSangranti: First day of Nepali Months e.g Maghe Sangranti Biska Jatra: First day of new year of Bikram Sambat