DAY_OF_WEEK_OFF_1 is an optional field which indicates the 1st preferred day off for this employee. e.g. Monday, Tuesday, Weekdays, Weekends, etc.

X_DAY_OF_WEEK is a fixed lexicon with the following values:
Saved
Value
Displayed Value
01 Sunday
02 Monday
03 Tuesday
04 Wednesday
05 Thursday
06 Friday
07 Saturday
08 Weekdays
09 Weekends
10 Week Start Day