Why do the dates of Muslim holidays in Uttarakhand change from year to year?

The dates of Muslim holidays in Uttarakhand, as in the rest of the world, shift from year to year because they are determined by the Islamic calendar, which is a lunar calendar.12

Unlike the solar-based Gregorian calendar, the Islamic calendar consists of 12 months based on the cycles of the Moon, resulting in a year of approximately 354 or 355 days. Because this lunar year is shorter than the solar year, there is no intercalation of months to keep the calendar aligned with the seasons. Consequently, the months and the holidays associated with them, such as Ramadan, Eid al-Fitr, and Eid al-Adha, retrogress through the solar year, shifting by about 11 days annually.2341

Furthermore, the start of each month is traditionally determined by the sighting of the new moon, which can lead to slight variations in the exact date of observance depending on local visibility and meteorological conditions.256