oneThis plural form is used for numbers like: 1
Repeat every %1$d day
otherThis plural form is used for numbers like: everything else
Repeat every %1$d days
repeat_every_n_days
Repeat every %1$d day
Repeat every %1$d days
Repeat every %1$d day
Repeat every %1$d days
%1$d - number, %1$d may be skipped for singular if translation looks better.