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First, a little theory is in order. The "hand" of the thread... whether it tightens clockwise or counterclockwise... is critical in designing a rotating device such as a saw, router or drill. In order for the mounting bolt to stay tight, the threads on the bolt must tighten "opposite" of the rotation of the blade... otherwise the turning of the blade would eventually cause the bolt to turn out! Therefore, if the blade on your saw rotates counterclockwise, then the bolt is tightened clockwise. So, if you blade turns counterclockwise, then loosen the bolt counterclockwise. Right-handed circular saws all have blades that turn counterclockwise... left-handed saw blades turn clockwise. My head's spinning... glug.
For ease of blade removal, many saws utilize a sliding lock or lever that holds the blade in place for loosening the bolt. Look for such a lever on the motor-side of the saw. If your saw does not have such a built-in blade lock, clamp a pair of Vise Grips or a clamp to the blade and turn the blade till it is locked in place. Then you can loosen the bolt.