The Hijri (Islamic) year is shorter than the solar year by about:

The Hijri (Islamic) year is a purely lunar year of 12 months, with about 354 or 355 days. The solar (Gregorian) year has 365 or 366 days. So the Hijri year is about 11 days shorter than the solar year.

Why the difference arises:

• Each Islamic month begins with the sighting of the new crescent moon, and one lunar month (from one new moon to the next) lasts about 29.5 days.
• 12 lunar months therefore add up to about 354.37 days.
• The Islamic calendar does not add an extra (intercalary) month to keep in step with the seasons, unlike the Hindu, Jewish and Chinese lunisolar calendars.

Effects of the difference:

Islamic festivals, such as Ramadan, Eid al-Fitr, Eid al-Adha and Muharram, move about 11 days earlier every year in the Gregorian calendar.

They pass through all the seasons in a cycle of about 33 solar years. For example, Ramadan may fall in winter in some years and in the hot summer in others.

About 33 Hijri years are roughly equal to 32 Gregorian years.

Other facts about the Hijri calendar:

• It counts years from the Hijrah, the migration of the Prophet Muhammad from Mecca to Medina in 622 CE.
• It was introduced by the second Caliph, Umar, around 638 CE.
• Its first month is Muharram and its ninth month is Ramadan.

Comparison with other calendars:

• Gregorian calendar: solar; introduced by Pope Gregory XIII in 1582 to correct the Julian calendar.
• Indian National Calendar (Saka): solar; adopted on 22 March 1957.
• Hindu calendars: lunisolar; they add an extra month called "Adhik Maas" (or "Purushottam Maas") about every 32-33 months to stay in line with the seasons.

Why not the other options?

• ✗5 days is too small; the gap is about 11 days.
• ✗19 days does not match the actual difference.
• ✗30 days is roughly the length of one lunar month, which lunisolar calendars add as an extra month every few years.