The term "September 11th Jewish holiday" functions grammatically as a noun phrase, where "September 11th" acts as an adjective modifying the noun "holiday." However, this noun phrase refers to a non-existent concept, as there is no fixed Jewish holiday that occurs annually on September 11th of the Gregorian calendar. Jewish holidays are determined by the Hebrew calendar, which is lunisolar. Consequently, the Gregorian date of any Jewish holiday varies from year to year.
The Hebrew calendar is based on both the cycles of the moon and the sun. A lunar month is approximately 29.5 days, making a lunar year of 12 months about 11 days shorter than a 365-day solar year. To ensure the holidays align with their proper seasons (e.g., Passover in the spring), the Hebrew calendar periodically adds a 13th leap month. This system causes the dates of Jewish holidays, such as Rosh Hashanah or Yom Kippur, to shift relative to the Gregorian calendar. For example, Rosh Hashanah can occur anytime from early September to early October.
While a Jewish holiday could, by coincidence, fall on September 11th in any given year, it is not a designated or recurring date for any observance. The query for a "September 11th Jewish holiday" likely stems from a misunderstanding of the fundamental difference between the fixed solar Gregorian calendar and the variable lunisolar Hebrew calendar. The relationship between the two systems is not static, making it impossible to assign a permanent Gregorian date to any Jewish festival.
