King Bhumibol Adulyadej looked frail on his last appearance before the general public on September 1, when he was taken on a brief tour of the Bangkok hospital where he has spent most of the last six years.
His most recent ailment was a lung infection that required him to be fed intravenously and use a machine to aid his breathing.
One major gathering point this year was outside the hospital that has become his de facto palace, where well-wishers came to offer their prayers.
The king's other medical issues in recent years have included excess fluid in the brain and an operation to remove his gallbladder.
A somber reminder of his generation's passing came this morning with the death of 96-year-old former Foreign Minister Siddhi Savetsila, a member of the Privy Council, the king's personal advisory board.
His intervention during major political crises is generally seen as having been key to restoring the status quo.
Before the decline in his health, Bhumibol's birthday had also been the occasion for a much-anticipated annual speech in which he would speak his mind to exercise his authority as the country's moral leader.
Love of the monarchy is almost seen as the definition of Thainess. But it is not so clear whether the people's strong devotion for the king will be transferred to his son and heir apparent, 63-year-old Crown Prince Vajiralongkorn, who does not have his father's record of public service.