The National Day is a designated date on which celebrations mark the nationhood of a nation or non-sovereign country. This nationhood can be symbolized by the date of independence, of becoming a republic or a significant date for a patron saint or a ruler (birthday, accession, removal, etc.). Often the day is not called "National Day" but serves and can be considered as one. The National Day will often be a national holiday.

Many countries have more than one National Day. For example, Pakistan has three National Days, none of which is named the "National Day". This signals the use of a "class" of National Days, that are equally important in the foundation of the nation, and a "class" of less important official public holidays.

Most countries have a fixed date National Day, but some have movable dates. An example is Jamaica, which up to 1997 celebrated its National Day on the first Monday in August. This commemorated independence from the United Kingdom which was attained on Monday, 6 August 1962, the first Monday in August of that year. Israel's Independence Day is not fixed relative to the civil Gregorian calendar, being formally linked to 5 Iyar on the Jewish calendar, but may also be moved to any day between 3 and 6 Iyar in order to avoid preparing for or celebrating either Independence Day or Memorial Day (which immediately precedes Independence Day) on Shabbat; the practical effect of this is that Independence Day may be celebrated any day between 15 April and 15 May. Another example is Thailand which celebrates the birthday of the king on 5 December. This date will change on the accession of the heir to the throne.

許多國(guó)家的國(guó)慶節(jié)是固定不變，不過(guò)也有國(guó)家的國(guó)慶節(jié)是變化的。牙買加就是一個(gè)例子，直到1997年開(kāi)始在8月的第一個(gè)星期一慶祝其國(guó)慶節(jié)。他們是為了慶祝1962年8月6日脫離英國(guó)獨(dú)立，當(dāng)時(shí)是星期一，而這個(gè)日子是那年8月的第一個(gè)星期一。以色列的獨(dú)立日根據(jù)他們本國(guó)的格列高利歷每年的日期也不同。通常是猶太歷的5伊雅爾月，但是為了避免在安息日里同時(shí)慶祝獨(dú)立日或紀(jì)念日（緊跟在獨(dú)立日的前面）也有可能變動(dòng)至3伊雅爾月和6伊雅爾月之間。結(jié)果就是以色列的獨(dú)立日慶典也許在4月15日至5月15日任何一個(gè)日子。泰國(guó)的國(guó)慶節(jié)也不是固定的，他們?cè)?2月5日慶祝國(guó)王的生日。但是這一日期隨著繼承人登基的日子不同而變化。

Importance attached to the National Day as well as the degree to which it is celebrated vary greatly from country to country. For example, Spain's National Day Fiesta Nacional de España is held on 12 October, the day celebrated in other countries as Columbus Day or Día de la Raza, and commemorates the anniversary of Christopher Columbus's arrival in the Americas. A military parade is held in Madrid celebrating the occasion. The National Day in France is 14 July and known as the Fête nationale (known outside France as Bastille Day) commemorating the Storming of the Bastille, which is considered the start of the French Revolution. It is widely celebrated and the French Tricolour is much in evidence, while the President of the Republic attends a military parade on the Champs-Élysées of Paris. In the United States, the Independence Day celebrations on 4 July are widely celebrated with parades, fireworks, picnics and barbecues. In Ireland, Saint Patrick's Day, 17 March, is the equivalent of a national day and has been a public holiday for many years. However, in the United Kingdom the constituent countries' patron saints' days are low-key affairs. In recent times campaigns have commenced to promote the National Days of England, Scotland and Wales, with St. Andrew's Day being designated as an official bank holiday when the Scottish Parliament passed the St Andrew's Day Bank Holiday (Scotland) Act 2007. A National Day for the United Kingdom has also been proposed in recent years.

各國(guó)國(guó)慶節(jié)的慶祝儀式都不盡相同，受重視程度也不同。例如，西班牙國(guó)慶節(jié)Fiesta Nacional de España在10月12日，這一天在別的國(guó)家則是哥倫布發(fā)現(xiàn)美洲紀(jì)念日（Día de la Raza），是紀(jì)念克里斯多夫·哥倫布來(lái)到美洲的周年慶典。馬德里會(huì)舉行閱兵式來(lái)慶祝國(guó)慶節(jié)。法國(guó)的國(guó)慶節(jié)則是7月14日，法語(yǔ)中稱為Fête nationale（法國(guó)之外的地方稱之為巴士底日），是紀(jì)念攻占巴士底獄，也是法國(guó)大革命的開(kāi)端。那天全法普天同慶，隨處可見(jiàn)法國(guó)三色旗，與此同時(shí)法國(guó)總統(tǒng)出席巴黎香榭麗舍大街的閱兵式。美國(guó)的國(guó)慶節(jié)也就是獨(dú)立日，每年的7月4日，他們通常會(huì)在那天游行、放煙火、野炊和燒烤以示慶祝。愛(ài)爾蘭3月17日的圣帕特里克日相當(dāng)于他們的國(guó)慶節(jié)，已作為國(guó)家假日很多年。但是大英帝國(guó)的各個(gè)聯(lián)合區(qū)域的守護(hù)神的節(jié)日沒(méi)有受到重視。最近多個(gè)運(yùn)動(dòng)開(kāi)始提升英格蘭、蘇格蘭和威爾士國(guó)慶節(jié)的關(guān)注度。蘇格蘭議會(huì)通過(guò)了2007年的《圣安德魯日休假法案》，于是圣安德魯日成為了法定節(jié)假日。