[SalesForce] Apex: How to generate List of all Friday Dates for given month

Update

Borrowing from @highfive's answer, the easiest way to do this is use the DAY_ONLY operator to convert the DateTime.

SELECT Id FROM Case WHERE DAY_ONLY(ClosedDate) >= :startOfMonth
  AND DAY_ONLY(ClosedDate) <= :endOfMonth

Old Answer

So it seems like there are four options. I'd probably go for the last one, which seems the clearest and simplest. In hindsight this question appears subjective and probably not right for the forum =.

First, you can use @sathya's suggestion which works but might be counter-intuitive if you don't understand the date to datetime conversion that's happening.

Date startOfMonth = Date.newInstance(2012,9,1);
Date endOfMonth = Date.newInstance(2012,10,1);
List<Case> cases = [
        select id from Case 
        where closedDate >= :startOfMonth 
        and closedDate <= :endOfMonth];

Second, you can exactly specify the date time window. Which works but feels a little verbose.

DateTime startOfMonthDT = DateTime.newInstance(2012,9,1,0,0,0);
DateTime endOfMonthDT = DateTime.newInstance(2012,9,30,23,59,59);
List<Case> cases = [
        select id from Case 
        where closedDate >= :startOfMonthDT 
        and closedDate <= :endOfMonthDT];

Third, if you're only worrying about the current or last month you can use date literals.

List<Case> cases = [
        select id from Case 
        where closedDate = LAST_MONTH];

Finally, you can use the date functions, which seems like the clearest (to me at least)

Integer month = 9;
Integer year = 2012;
List<Case> cases = [
        select id from Case 
        where CALENDAR_MONTH(closedDate) = :month
        and CALENDAR_YEAR(closedDate) = :year];

As far as how that function can be used in a query, it would look something lie:

SELECT Id FROM MyObject__c WHERE CreatedDate = THIS_MONTH
AND DAY_IN_MONTH(CreatedDate) IN (3, 10, 17, 24, 31)

However, you could instead use DAY_IN_WEEK and save yourself the trouble of calculating which dates you need:

WHERE DAY_IN_WEEK(CreatedDate) = 6

Note that 1 represents Sunday, and 7 represents Saturday. Since Saturday - 1 = Friday, you should use a value of 6

If you wanted to query records created on Fridays in a particular month/year, it would look like:

public static List<MyObject__c> getRecordsCreatedFriday(Integer year, Integer month)
{
    return [
        SELECT ... FROM MyObject__c
        WHERE CALENDAR_YEAR(CreatedDate) = :year
        AND CALENDAR_MONTH(CreatedDate) = :month
        AND DAY_IN_WEEK(CreatedDate) = 6
    ];
}

You could also add an overload that accepts just a date within the month to filter:

public static List<MyObject__c> getRecordsCreatedFriday(Date dateInMonth)
{
    return getRecordsCreatedFriday(dateInMonth.year(), dateInMonth.month());
}

It might just be worth reading up on all Date Functions.