How To Set Shedule Messages In Whatsapp 
All you have to do is go to the Google Play Store and install a third party app
The message will be automatically sent to the WhatsApp contact at its scheduled time
Whether it's video calling with friends or doing business on the phone, WhatsApp is on the rise. The company is also rolling out new features to increase user engagement. However, users are still waiting for the Schedule Message Feature.
    
Even if the company has not recently added this feature to WhatsApp, there are other ways you can send a scheduled message to WhatsApp. This trick is very useful if you want to wish someone a birthday or send a message regarding a meeting at 12 o'clock at night. Let's get started.

This is how messaging can be scheduled in Android smartphones
Step 1: For this go to Google Play Store, download third party app SKEDit, open the app and sign up.
                                          
Step 2: Login and tap on the WhatsApp option provided in the main menu. You will now be asked for some permissions.
                                  
Step 3: Now you have to click on Enable Accessibility. Then tap on Use service.
                                     
Step 4: Now search the name of the contact you want to send the scheduled message to in WhatsApp chat, type the message and set the time and day.
                                         
Note: You can also decide if you want to repeat the scheduled message. Here you will also find the option to Ask me before sending. After submitting all the information, click OK.
                                           
Step 5: Now your message will be automatically sent to WhatsApp at the specified time.
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SKEDit Scheduling App:
SKEDit is a free and easy-to-use scheduling app that lets you schedule WhatsApp messages, schedule SMS, send emails later, schedule posts, and set call reminders. This top-rated Android scheduling app acts like your personal virtual assistant. Put your communication on autopilot with SKEDit, the free 5-in-1 WhatsApp scheduler, SMS scheduler, Call scheduler, and Email scheduler.

DISCLAIMER: Some services are not compatible with all devices (especially older ones with older OS), and require strict requirements for their automation to function properly (e.g removing screen lock- more details in help center), as the service itself does not provide any automation capabilities (hence the “macro” workaround). Thank you for your understanding.

+ Save Time: Schedule WhatsApp messages, SMS, and emails to be sent later automatically. Save countless hours lost in tedious back-and-forth messaging each day.
+ Eliminate Stress: Streamline all your communication so it runs smoothly. Calmly breeze through your day not worrying about sending message reminders or last-minute phone calls.
+ Stay Organized: Structure, schedule & plan all your communication down to a T. No more drowning in an ocean of unwelcomed phone calls, emails, and messages.
+ Boost Productivity: All your communication needs handled. Tackle your top priorities with laser-like focus with a fully-free WhatsApp, post, and SMS scheduler.
+ Automate Your Communication: Swift, automated communication makes everything fall into place like clockwork. Relax as you sail through your day, having already planned it yesterday.
+ Set Reminders: Schedule WhatsApp messages, automatic emails, and call reminders to be sent to yourself. Enjoy effortlessly staying in-sync with your plans through the power of automated reminders.

HOW IT WORKS - 4 EASY STEPS
[1] Quickly Register: Simply fill in contact info, or use One-Touch Facebook register, for a rapid download & install.
[2] Choose Communication Service: Select communication method you’d like to automate: Call, SMS, Email, WhatsApp
[3] Write Content: Jot down your message or content you’d like to have automatically sent
[4] Schedule Date & Time: Set exact time & date for your message, content or call. Sit back & relax while SKEDit handles the rest

POWERFUL SCHEDULING APP THAT ALLOWS YOU TO CONTROL YOUR TOMORROWS, TODAY
SKEDit combines human power with the precision of AI automation, to make your future life simpler, easier and stress-free. This nifty little scheduling app belongs in your bag of tricks if you’re / you’re a:

+ Person who needs help remembering birthdays/special events of loved ones
EG: schedule WhatsApp messages or schedule SMS to be sent later at the best time
+ Busy with other activities but still need to communicate
EG: schedule automatic emails to be sent when you know you’ll be caught in long meetings
+ Always needing to remind others
EG: schedule Whatsapp messages using the Whatsapp scheduler to remind your family of important meet-ups, to-do chores or set call reminders to stay in touch.
+ Blogger using many different communication channels with fans
EG: schedule posts or schedule messages so they appear routinely at the same times
+ Businessman travelling many different time zones
EG: schedule WhatsApp messages or automatic emails out to your team before you travel
+ Always driving & wanting to avoid texting/calling, but still needing to communicate
EG: schedule SMS using the SMS scheduler for when you know you’ll be driving so you’re not constantly glancing at your phone and putting yourself in danger on the road

For iPhone detail steps :: 
There is no app for smartphones like Android, but you can do this with the help of Siri and shortcut apps.
If the phone does not already have a shortcut app, download it from the App Store.
Click on the Automation tab at the bottom.
Tap on the + icon on the top right corner and tap on Create Personal Automation.
Now tap on Time of Day to schedule the message. Select the day and time and tap Next.
Tap Add Action and type Text in the search bar and select the Text option from the list.
Now type the message. Search for WhatsApp again by tapping on the + icon below the text field.
Now select Send Message via WhatsApp from the action list. Select the contact and tap Next.
Now your message will automatically go to WhatsApp at the scheduled time.