How to trigger a Lambda function at specific time in AWS?

Using cloud watch rules - a single non-repetitive date can be assigned.

Example cron : 0 10 16 6 ? 2019 min hours day month dayOfWeek year

This will execute once on Sun, 16 Jun 2019 10:00:00 GMT as an example.

Reference: https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/ScheduledEvents.html

you can provide to it a cron expression or rate. what you are looking for is the cron expression option that will let you to say when exactly to run. more info - https://docs.aws.amazon.com/lambda/latest/dg/tutorial-scheduled-events-schedule-expressions.html

if you want to run it once, manually, you can always trigger it with the "test" button at the top of the page. you can really be creative here, for example, you can even trigger it with an http request that will allow you to integrate it to whatever you want, super easily (Invoke a AWS Lambda function by a http request)

You will need to make use of cloudwatch event rules. Lambda can be triggered from the rule based on the cron expression you provide. This AWS tutorial will walk you through the setup.