Find timestamp for highest number of concurrent users by date
In PostgreSQL you can simply use DISTINCT ON:
SELECT DISTINCT ON (concurrent_date) *
FROM   sample_data
ORDER  BY concurrent_date, concurrent_users DESC;

We don't need the redundant column concurrent_date for this at all. Casting concurrent_timestamp on the fly is very cheap - overall cheaper than storing the functionally dependent value redundantly.
SELECT DISTINCT ON (concurrent_timestamp::date) *
FROM   sample_data
ORDER  BY concurrent_timestamp::date, concurrent_users DESC;

db&lt;&gt;fiddle here
See:

Select first row in each GROUP BY group?

DISTINCT ON is typically fastest for few rows per group. Depending on table definition and data distribution, there may be (much) faster solutions. See:

Optimize GROUP BY query to retrieve latest row per user