I have a 10 year Multiple entry visit visa to USA issued in April 2019. By what date shall I make the first entry to USA to keep the visa valid ?
Your visa is already valid and does not need to be activated by travel. You do not need to keep to the original travel plans you described in your visa application. You can change or cancel these plans if necessary.
You can enter at any time on or before the expiry date of the visa. The expiry date is the last day that you can enter the US, not the date that you must exit. You can enter the US on the visa expiry date and be admitted for a full six months. Each time you enter the US, your passport will be stamped with the date of entry and the date you must exit. You need to leave the US before the end of the day of that date.
Each time you travel to the US, you must be going for one of the purposes permitted to tourists or business visitors (B1/B2). The visa allows you to travel to the US as many times as you wish, but it does not guarantee that you will be allowed to enter. You still must satisfy the officer at the border that you will adhere to the rules and leave the US without overstaying.
If your passport expires, the visa still remains valid. Travel with both your old and new passports in this case. When you renew your passport, be sure to ask your country's passport authority to return the old passport.
The main offical site, shown below, does not meantion a first date of usage requirement.
Only that the passport must be valid 6 months after the expected exit and that an expired passport with a valid visa can be used with a new passport.