This formula uses the WEEKDAY function to test dates for either a Saturday or Sunday. When given a date, WEEKDAY returns a number 1-7, for each day of the week. In it's standard configuration, Saturday = 7 and Sunday = 1. By using the OR function, use WEEKDAY to test for either 1 or 7. If either is true, the formula will return TRUE and trigger the conditional formatting.
Highlighting the entire row
If you want to highlight the entire row, apply the conditional formatting rule to all columns in the table and lock the date column: