Simply cast your timestamp AS DATE, like this:
SELECT CAST(tstamp AS DATE)
In other words, your statement would look like this:
SELECT SUM(transaction_amount)
FROM mytable
WHERE Card_No='123'
AND CAST(transaction_date AS DATE) = target_date
What is nice about CAST is that it works exactly the same on most SQL engines (SQL Server, PostgreSQL, MySQL), and is much easier to remember how to use it.
Methods using CONVERT() or TO_DATE() are specific to each SQL engine and make your code non-portable.