Some things I dug up while researching my novel Beethoven's Assassins.

Beethoven in Fiction
When was Beethoven born?
What caused his deafness?
Who was the “Immortal Beloved”?
Why "Moonlight" Sonata?
Was Beethoven a Freemason?
What was the "Incident at Teplitz"?
Why "Hammerklavier" Sonata?
Who first played the Hammerklavier Sonata?
Bibliography

When was Beethoven born?

Most authorities accept December 16th 1770 as Beethoven’s birth date, though the only record is of his baptism on 17th December 1770. Can it be assumed that he was baptised on the day following his birth? According to Thayer-Krehbiel,¹ “[Since the] custom obtaining at the time in the Catholic Rhine country [was] not to postpone the baptism beyond 24 hours after the birth of a child, it is in the highest degree probable that Beethoven was born on December 16, 1770.” Yet the same source noted an entry by Beethoven’s nephew in a conversation book in 1823: “To-day is the 15th of December, the day of your birth, but I am not sure whether it is the 15th or 17th, inasmuch as we can not depend on the certificate of baptism...”²

If the 24-hour rule is correct, Beethoven might have been born on the 16th or 17th December. In 1838, Beethoven’s friend Franz Wegeler gave the 17th as Beethoven’s birthday.³ Yet in 1906, A.C. Kalischer disputed the 24-hour rule, saying, “Up to three days may elapse after birth.”⁴ Citing the nephew’s conversation-book entry, Kalischer said the 15th of December was “far more probable as Beethoven’s birthday than the 16th or even the 17th of December”.

Moreover, for much of his life, Beethoven thought his birth year was 1772. When he received a copy of his baptismal certificate for the first time in 1810, he wrote on the back, “1772. The baptismal certificate seems to be incorrect, since there was a Ludwig born before me.”⁵ That Ludwig van Beethoven, baptised April 2, 1769, lived only six days, and was no cause for confusion.⁶ It’s not clear why Beethoven thought himself two years younger than his real age. A plausible explanation would be parental efforts to promote him as a child prodigy, but biographer Maynard Solomon rejected this, instead considering Beethoven’s error a form of delusion.⁷

Beethoven seems to have regarded the 16th as his birthday, at least in his later years. Theodore Albrecht, translator and editor of Beethoven’s conversation books, surmised that an entry on December 17th 1819, by a hungover Franz Oliva, indicated celebration at Beethoven’s apartment the previous evening.⁸

References

1. Thayer-Krehbiel I, p53
2. Thayer-Krehbiel I, p53 n1
3. Wegeler-Ries, p9
4. Wegeler-Ries, p171
5. Thayer-Forbes, p54
6. Thayer-Forbes, p52
7. Solomon, p4
8. Conversation Books Vol 1, p141