Carry out the following steps to create an Opportunity close date email alert using a workflow:

1. Click on the Setup gear icon in the top right-hand corner of the main Home page, as shown in the following screenshot:

2. Click on Setup, as shown in the following screenshot:

3. Navigate to the workflow rule customization setup page as follows: Process Automation | Workflow Rules.
4. Click on Continue (if shown).

The Continue button is found on the Understanding Workflow page (if this is shown), as shown in the following screenshot:

5. Click on New Rule.

We will be presented with the Step 1: Select object page.

6. Select Opportunity.
7. Click on Next, as shown in the following screenshot:

We will be presented with the Step 2: Configure Workflow Rule page.

8. Enter Alert Opportunity Close Date in the Rule Name field.
9. Optionally, enter a description in the Description field.
10. Select the created, and any time it's edited to subsequently meet criteria option from the Evaluate Criteria options list.
11. In the Rule Criteria section, set the Run this rule if the following option to formula evaluates to true.
12. Paste or enter the following formula:

NOT(IsClosed)

13. Click Save & Next, as shown in the following screenshot:
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We will be presented with the Step 3: Specify Workflow Actions page.

14. In the Time-Dependent Workflow Actions section, click the Add Time Trigger button, as shown in the following screenshot:

15. We will be presented with the Add Time Trigger page.
16. Enter 21.
17. Choose Days.
18. Choose Before.
19. Choose Opportunity: Close Date.
20. Click on Save, as shown in the following screenshot:

We will be presented with the Step 3: Specify Workflow Actions page.

21. In the Time-Dependent Workflow Actions section, set the Add Workflow Action option to New Email Alert, as shown in the following screenshot:

22. We will be presented with the Email Alert Edit page.
23. Enter Alert Opportunity Close Date in the Description field.
24. Accept the default Alert_Opportunity_Close_Date text that is automatically set in the Unique Name field.
25. Now, choose the Alert Opportunity Close Date template that we created in the Getting ready section.

26. Set the Recipient Type to Owner.
27. Set the Selected Recipients to Opportunity Owner.
28. Click on Save, as shown in the following screenshot:

We will be presented with the Step 3: Specify Workflow Actions page, as shown in the following screenshot:

29. Click on Done.

We will be redirected to the new workflow rule where we will activate the rule, as shown in the following screenshot:

30. Click on Activate.

If there is no default workflow user currently set, a user will need to be chosen and you will be alerted with a warning message, as shown in the following screenshot:

31. Click on OK.

If there is no default workflow user currently set we will be redirected to the Process Automation Settings page, where we can set the Default Workflow User, as shown in the following screenshot:

32. Set the Default Workflow User.
33. Click on Save.

We are now, finally, redirected to the Workflow Rule Detail page with the Active checkbox set, as shown in the following screenshot: