Check the manual for the date command. Really depends on the format the date is inputed. the '%w' variable for date formating returns the 'day of the week' where 0 is sunday and 6 is Saturday.
In general you can use something like the following.
Code:
echo "Please enter a date YYYY/MM/DD"
read date
day=`date -d $date +"%w"`
if [ $day -eq 6 ]
then
echo "$date is a Saturday"
else
echo "$date is NOT a Saturday"
fi
