The name is glossed as to lift or to raise. It is not one of the four sacred months. The twelve month names are Arabic and predate Islam, retained from the calendar of the Arabian peninsula when the Hijri reckoning was established.
Four of the twelve are the sacred months, al-ashhur al-hurum, in which warfare was forbidden in pre-Islamic Arabia and which the Quran confirms at sura 9 verse 36: Dhu al-Qadah, Dhu al-Hijjah, Muharram and Rajab. Three run consecutively and the fourth stands alone in the middle of the year, an arrangement that fitted the pilgrimage season and the trading truces that went with it.
The Hijri calendar admits no intercalation. The verse immediately following the one that names the twelve months condemns nasi, the pre-Islamic practice of postponing or adding a month, and the calendar has run as a strict lunar reckoning since. The consequence is a year of 354 or 355 days that moves against the seasons by about eleven days annually, so any month passes through the whole solar year in roughly thirty-three years.
Shawwal opens with Eid al-Fitr on the first, the festival that ends the fast of Ramadan, and the sighting of the crescent that begins the month is therefore among the most closely watched observations in the calendar. Differences of sighting between countries produce differences in the date of the festival.