Its approximately 63-65 days after being bred - so if it was April 21 she could have them any day. Just curious as to WHY you are happy your cat is pregnant - doesn't sound like she's a purebred?
If she's not in a legit breeding program, then please have her spayed as soon as your vet will do it and have her kittens also spayed/neutered too before they leave.
Keep in mind the ideal time to adopt the kittens out is 10-12 weeks old - any sooner and you can have problems with the kittens adjusting physically/emotionally. They need this critical time with mom/siblings to learn more then eating and using the litter pan