first letter; first correspondence
Refers to the first letter or message received in a new year, season, or other period. Often used in a literary or slightly formal context.
Thank you for your first letter of the new year.
The first letter of spring has arrived.
Compound of 初 (hatsu, 'first') and 便り (tayori, 'letter, correspondence'). The reading はつだより follows rendaku voicing of たより to だより.