"I've been working in the Forbidden City for 33 years, and I thought I was quite familiar with this place," Chen said.

"But when we began to dig out behind-the-scenes stories to build up a multidimensional image of ancient China's hub of imperial power, it felt like I got to know it again," she said.

Chen added that such a calendar connects people's fragmented knowledge of the Palace Museum into a larger, more-integrated picture.

For the Forbidden City, publishing calendars is not a new idea.

Wang said the staff once tried it decades ago to make the Forbidden City more approachable to the public. The compound where 24 emperors from the Ming (1368-1644) and Qing (1644-1911) dynasties once lived became no longer "forbidden" after being named a national museum in 1925.

Between 1933 and 1937, the museum released calendars, but the project was halted after Beijing was occupied by the Japanese during the War of Resistance Against Japanese Aggression (1931-45).

This practice had almost been forgotten until Zheng Xinmiao, director of the museum from 2002 to 2012, happened across some old calendars.

The tradition was resumed in 2009, using a format identical to that used in 1937. The unexpected popularity of the annual revival provided museum staff greater confidence for creativity. Since 2010, a new version of calendars has been released annually and over 3 million have been sold since 2009, according to the museum.

As people's lives get more digitized, paper calendars have largely lost their functionality. Nevertheless, the museum seems to have led to a revival.

"People's nostalgia for recording time and their pursuit of beauty will never waver," Chen said.

Nie Zhenning, chairman of Taofen Foundation, a charitable organization, said there were only four kinds of thematic calendar books in 2012 nationwide. But the number rocketed to over 200 in 2018, spanning natural science, fine arts, health and more.

"The Palace Museum calendar has created a unique genre in China's publishing industry," Nie said.