The ninth month of the Muslim calendar is Ramadan, a period of obligatory fasting in commemoration of Muhammad's receipt of God's revelation. During the month all but the sick, the weak, pregnant or lactating women, soldiers on duty, travelers on necessary journeys, and young children are enjoined, as appropriate to their state in life, from eating, drinking, smoking, and sexual intercourse during daylight hours. The wealthy usually do little or no work during this period, and some businesses close for all or part of the day. Since the months of the lunar calendar revolve through the solar year, Ramadan falls at various seasons in different years. Summertime fasting imposes considerable hardship on those who must do physical work. Id al Fitr, a feast celebrated throughout the Islamic world, marks the end of the month of fasting. Gifts, the wearing of new garments, exchanges of sweetmeats, almsgiving, and visits to friends and relatives are some of the customs of this great religious festival.