Nothing simple exists that I'm aware of.
However, check out the useful advanced formulae page: http://login.salesforce.com/help/doc/en/useful_advanced_formulas.htm
In particular, this has a formula for calculating the day of the week as a number (0 = Sunday...6=Sunday).
MOD(entry_date__c - DATE(1900, 1, 7), 7)
The above will give you Entry Date's day of the week. To get Sunday, you should be able to subtract the result of that from the Entry Date
DATE(
YEAR(Entry_Date__c),
Month(Entry_Date__c),
DAY(Entry_Date__c)-MOD(entry_date__c - DATE(1900, 1, 7), 7))
Though you would need to add some complexity to this to handle the start of a month/year.
Edit:As @TomLogic points out below, you may be able to avoid using the DATE function and having to add complexity to handle start of month/year issues by subtracting the result of the Mod operation from the Date directly (though I've not yet tested it!)
Entry_Date__c - MOD(entry_date__c - DATE(1900, 1, 7), 7))
Best Answer
You mean the 18 digit Id? You can use CASESAFEID(Id) as a formula to return the 18 digit case insensitive version of the record's Id.