Best workout to get a six pack by the end of July or summer vacation (august)?

Don't just do regular situps. If you want defined abs then you also want to work the rest of your core, mainly your obliques. To work on those just do a normal situp but twist when you get to the top, in either direction. Alternate directions each time you go up. If you do 150 regular situps, do 300 twist situps (150 for each side). Nutrition is key though. Make sure you alter your diet to aid in sculpting your abs. Check out bodybuilding.com for more workout and nutrition tips! They have a special diet plan just for building ripped abs!