When?

Thanksgiving Day
fourth Thursday in November in the United States; second Monday in October in Canada; commemorates a feast held in 1621 by the Pilgrims and the Wampanoag
 
tide
there are usually two high and two low tides each day
 
Adar
the sixth month of the civil year; the twelfth month of the ecclesiastic year in the Jewish calendar (in February and March)
 
Passion Sunday
second Sunday before Easter
 
Pascha
the Jewish feast of the Passover
 
red-letter day
a memorably happy or noteworthy day (from the custom of marking holy days in red on church calendars)
 
Fast of Gedaliah
(Judaism) a minor fast day on Tishri 3 that commemorates the killing of the Jewish governor of Judah
 
American Indian Day
US: the 4th Friday in September
 
prophase
the first stage of mitosis
 
Christian year
the year in the ecclesiastical calendar; especially feast days and special seasons
 
Whitsun Monday
the day after Whitsunday; a legal holiday in England and Wales and Ireland
 
Fast of Esther
(Judaism) a minor fast day on Adar 13 commemorates three days that Esther fasted before approaching the Persian king on behalf of the Jewish people; the fast is connected with Purim
 
lustrum
a period of five years
 
eighties
the time of life between 80 and 90
 
Ramanavami
Hindu lunar holiday (on the 9th day of Caitra) to celebrate the birth of Rama
 
birthday
an anniversary of the day on which a person was born (or the celebration of it)
 
bloom
the best time of youth
 
Dominion Day
a legal holiday in Canada commemorating receiving Dominion status in 1867
 
Nivose
fourth month of the Revolutionary calendar (December and January); the snowy month
 
spare time
time that is free from duties or responsibilities
 
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