The word is
SOLSTICE, which is especially appropriate for this season! Although in my time zone the solstice was last night, rather than today :)
ERGO and HENCE are both synonyms of SO
LIGHTSECOND is abbreviated LS
TI is "a drink with JAM and BREAD" in The Sound of Music
CE, cerium, is ELEMENT FIFTY-EIGHT
The puzzle's title refers to
the summer solstice and winter solstice, respectively the longest and shortest days (time from sunup to sundown) of the year.