Well-wishers hold pictures of Thai King Bhumibol Adulyadej as they gather to celebrate his 86th birthday near Klai Kangwon Palace, Hua Hin, Prachuap Khiri Khan province, Dec. 5, 2013. Reuters

Millions of people in Thailand will flock Saturday to the streets to celebrate the king's 88th birthday. A revered figure in the country, King Bhumibol Adulyadej ascended the throne in 1946 -- making him the world's longest reigning head of state.

Each year, the day encompasses a range of celebrations across the country, from charitable events to athletic activities. Cities such as Bangkok and Hua Hin see massive crowds, with people traveling all from all over for the national holiday. In some cases, people walk long distances to these cities to raise money for charity, according to Thailand Today. Those in the country's capital of Bangkok gather outside the Royal Palace to catch a glimpse of King Bhumibol.

Here's what you need to know about the countrywide holiday:

1. The king's birthday is also celebrated as Thailand's Father's Day. The day is recognized as such because the king is considered the nation's ceremonial father. People may offer their fathers and grandfathers a "dok Buddha ruska," or canna flower, to show their appreciation.

2. Homes and public places are elaborately decorated with flags and portraits of the king to mark the occasion.

3. Religious ceremonies are held to honor the king. This year, one of the ceremonies will be led by Crown Prince Maha Vajiralongkorn. At some gatherings, people hold up lit candles in unison to pay homage to the king.

5. The king's birthday was declared by the United Nations General Assembly to be World Soil Day, in order to recognize his achievements raising awareness about soil's importance. Events are held internationally in cities such as Rome, New York and Bangkok to promote more sustainable uses of soil.

5. People traditionally wear the color yellow -- the royal color. The color is associated with Mondays, the day of the week when King Bhumibol was born.

Here is footage from last year's celebrations: