MySQL 8.0

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WITH calendar AS (
    SELECT 1 AS sno, '2019-01-01' AS datefield UNION ALL
    SELECT 2, '2019-01-02' UNION ALL
    SELECT 2, '2019-01-03' UNION ALL
    SELECT 2, '2019-01-04' UNION ALL
    SELECT 2, '2019-01-05' UNION ALL
    SELECT 2, '2019-01-06' UNION ALL
    SELECT 2, '2019-01-07' UNION ALL
    SELECT 2, '2019-01-08' UNION ALL
    SELECT 2, '2019-01-09' UNION ALL
    SELECT 2, '2019-01-10' UNION ALL
    SELECT 2, '2019-01-11' UNION ALL
    SELECT 2, '2019-01-12' UNION ALL
    SELECT 2, '2019-01-13' UNION ALL
    SELECT 2, '2019-01-14' UNION ALL
    SELECT 2, '2019-01-15' UNION ALL
    SELECT 2, '2019-01-16' UNION ALL
    SELECT 2, '2019-01-17' UNION ALL
    SELECT 2, '2019-01-18' UNION ALL
    SELECT 2, '2019-01-19' UNION ALL
    SELECT 2, '2019-01-20' UNION ALL
    SELECT 2, '2019-01-21' UNION ALL
    SELECT 2, '2019-01-22' UNION ALL
    SELECT 2, '2019-01-23' UNION ALL
    SELECT 2, '2019-01-24' UNION ALL
    SELECT 2, '2019-01-25' UNION ALL
    SELECT 2, '2019-01-26' UNION ALL
    SELECT 2, '2019-01-27' UNION ALL
    SELECT 2, '2019-01-28' UNION ALL
    SELECT 2, '2019-01-29' UNION ALL
    SELECT 2, '2019-01-30' UNION ALL
    SELECT 2, '2019-01-31'
),
final_registration AS (
    SELECT 1 AS id, '101' AS event_id, 'Ritu' AS name, '2019-01-15 13:31' AS booking_date UNION ALL
    SELECT 2, '101', 'Seeta', '2019-01-15 18:21' UNION ALL
    SELECT 3, '101', 'Geeta', '2019-01-16 13:21' UNION ALL
    SELECT 4, '102', 'Wasim', '2019-01-16 14:21' UNION ALL
    SELECT 5, '102', 'Rahul', '2019-01-17 13:21' UNION ALL
    SELECT 6, '101', 'Gagan', '2019-01-17 14:21' UNION ALL
    SELECT 7, '101', 'Sunny', '2019-01-17 15:21' UNION ALL
    SELECT 8, '101', 'Aman',  '2019-01-17 16:21'
)

SELECT
    c.datefield,
    COUNT(f.booking_date) AS cnt
FROM calendar c
LEFT JOIN final_registration f
    ON c.datefield = DATE(f.booking_date) AND
       f.event_id = '101'
WHERE
    c.datefield BETWEEN DATE_FORMAT(NOW() ,'%Y-%m-01') AND CURDATE()
GROUP BY
    c.datefield;

datefield	cnt
2019-01-01	0
2019-01-02	0
2019-01-03	0
2019-01-04	0
2019-01-05	0
2019-01-06	0
2019-01-07	0
2019-01-08	0
2019-01-09	0
2019-01-10	0
2019-01-11	0
2019-01-12	0
2019-01-13	0
2019-01-14	0
2019-01-15	2
2019-01-16	1
2019-01-17	3