oneThis plural form is used for numbers like: 1
In %1$d day
otherThis plural form is used for numbers like: everything else
In %1$d days
Dans %1$d jour
Dans %1$d jours
In %1$d day
In %1$d days
In %1$d day
In %1$d days
Dans %1$d jour
Dans %1$d jours
%1$d - number of days, %1$d may be skipped for singular if translation looks better.