Sindarin 

girithron

noun. December, *Shuddering-one

Sindarin [LotR/1110; SA/girith] Group: Eldamo. Published by

girithron

noun. december (month)

Sindarin [LotR/D] Group: SINDICT. Published by

girithron

Girithron

The name apparently contains girith "shivering" and raun "moon, month".

Sindarin [Tolkien Gateway] Published by

girithron

december

Girithron (na **Irithron**)

girithron

december

(na ’Irithron)