Any interest paid and applied to your mortgage in the current year is allowed as a deduction. However, see the "prepaid interest" note below from the IRS.
If you payments are designated for future months there is a limit to the amount you can deduct.
IRS Publication 936-Home
Mortgage Deduction (Click the link for more information)
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Prepaid interest. If you pay interest in advance
for a period that goes beyond the end of
the tax year, you must spread this interest over
the tax years to which it applies. You can deduct
in each year only the interest that qualifies
as home mortgage interest for that year.
