Ve a la ayuda de delphi, en la pestaña "Indice", introducen en el espacio de búsqueda esto "Datetime routines" dale mostrar cuando se encuentre, y se te mostrará una lista de funciones, entre las cuales está lo que buscas, estas son algunas
Day of week constants
Provide symbolic constants for ISO 8601-compliant day of the week values.
DayOf function
Returns the day of the month represented by a TDateTime value.
DayOfTheMonth function
Returns the day of the month represented by a TDateTime value.
DayOfTheWeek function
Returns the day of the week represented by a TDateTime value.
DayOfTheYear function
Returns the number of days between a specified TDateTime value and December 31 of the previous year.
DayOfWeek function
Returns the day of the week for a specified date.
DaysBetween function
Returns the number of whole days between two specified TDateTime values.
DaysInAMonth function
Returns the number of days in a specified month of a specified year.
DaysInAYear function
Returns the number of days in a specified year.
DaysInMonth function
Returns the number of days in the month of a specified TDateTime value.
DaysInYear function
Returns the number of days in the year of a specified TDateTime value.
DaySpan function
Returns the number of days (including fractional days) between two specified TDateTime values.
DecodeDate procedure
Returns Year, Month, and Day values for a TDateTime value.
DecodeDateDay procedure
Returns the year and day of the year for a specified TDateTime.
DecodeDateFully function
Returns Year, Month, and Day, and Day-of-Week values for a TDateTime value.
DecodeDateMonthWeek procedure
Returns the year, month, week of the month, and day of the week for a specified TDateTime.
DecodeDateTime procedure
Returns Year, Month, Day, Hour, Minute, Second, and Millisecond values for a TDateTime.
DecodeDateWeek procedure
Returns the year, week of the year, and day of the week for a specified TDateTime value.
DecodeDayOfWeekInMonth procedure
For a given TDateTime value, returns the year, month, day of the week, and the count of that day of the week in the month.
DecodeTime procedure
Breaks a TDateTime value into hours, minutes, seconds, and milliseconds.