Manmohan Singh at work in US on his birthday

New Delhi: Prime Minister Manmohan Singh, a famously reluctant birthday boy, turns 79 on Monday either in the high skies or in Frankfurt on his return journey to India after a hectic six-day foreign trip, say officials, adding that one thing was for sure - the celebrations would be extremely low-key.

"The Prime Minister will either be in Frankfurt or on board Air India One from New York to Frankfurt on his return journey from the 66th United Nations General Assembly," an official in the Prime Minister's Office (PMO) said.

Manmohan Singh was born in 1932 in Gah Begal village in what now lies in Punjab province of present-day Pakistan. During partition, his family shifted to India. Though September 26 is celebrated as his birthday, like many others born in villages, his actual date of birth remains a mystery, said an official.

"September 26 is shown in the school records as my date of birth," Manmohan Singh has said when asked about it.

An aide of the prime minister said his birthdays are known for their simplicity. "As a matter of principle, he believes in simplicity and avoids pomp and show on all occasions. The same goes for his birthdays," he added.

In 2010, Manmohan Singh was in India on his birthday. But on more than one occasion he has spent the day working abroad, especially in connection with the UN General Assembly, the aide said.

In 2004 and 2008 also, the prime minister was overseas on September 26, he recalled.

A workaholic who puts in 12-16 hours of work daily, Manmohan Singh attends office on his birthdays, when in India. "Last year too he attended office," the aide recalled.

"The only function in office is that we greet him and present him a joint birthday card," an official in the PMO said.

Manmohan Singh otherwise keeps celebrations limited to the family, spending the day with kith and kin. His wife Gursharan Kaur usually visits a gurdwara and offers special prayers on the day.

"But he avoids accompanying her, as it will create traffic jams and delay Delhi residents," the aide said.

Harish Khare, now media advisor to the man, was a journalist on board with Manmohan Singh on his first birthday as prime minister in 2004 when he came to head the United Progressive Alliance (UPA)-I government. The media and airline crew celebrated it on Air India One.

According to Khare, the crew had readied a cake, but they themselves cut it as a reluctant Manmohan Singh reached late. A journalist, Vishwa Bandhu Gupta, gifted him a birthday greeting balloon, he recalled.

According a close aide of the prime minister, the recent political turbulence triggered by corruption scandals against some UPA ministers has not frustrated him.

"He keeps his cool and continues working," the aide said. adding, "One call that Manmohan Singh may await September 26 may be from Congress president Sonia Gandhi."

Journalists accompanying him are looking forward to spending the day with him on board Air India One.

A journalist accompanying the Prime Minister said, "We hope he is on board when September 26 begins. There will be yet another unusual birthday venue and we'll have an off-beat story too!"