How can I conditionally format cells in Google spreadsheets containing dates to have a different background according to the day of the week, e.g highlight the Saturdays and Sundays in a column containing the days of the month?
This is possible with a script. Go to Tools → Script editor... and paste in this script:
Save the script (give the project a name) and go back to your spreadsheet.
Now, every time you edit a cell, this script will check to see if you entered a date and if the date you entered is a Sunday or Saturday. If so, the background of the cell will change to red. If it is not a date or it is not on the weekend, the background will change to white.
There are other color names you can use, or you can use a hex code to specify a different color.