It accepts argument "date" and returns appointments for that date.The date arg should be in "Y-m-d" format. Used with Appointments+ plugin

appointments_get_appointments_for_day.php

<?php
/*
Description: Copy file in mu-plugins folder. To be used with Appointments+ plugin. It accepts argument "date" and returns appointments for that date.The date arg should be in "Y-m-d" format.
*/

if( ! function_exists( 'appointments_get_appointments_for_day' ) ){

	function appointments_get_appointments_for_day( $args = array() ) {

		if( ! class_exists( 'Appointments' ) ){
			return;
		}

		global $wpdb;

		$table = appointments_get_table( 'appointments' );

		$defaults = array(
			'worker' => false,
			'service' => false,
			'location' => false,
			'user' => false,
			'date_query' => array(),
			'app_id' => array(),
			'status' => false,
			'per_page' => -1,
			'page' => 1,
			's' => false,
			'orderby' => 'ID',
			'order' => 'ASC',
			'count' => false, // Will return only the number of rows found
			'date' => date( 'Y-m-d' )
		);

		$args = wp_parse_args( $args, $defaults );

		$cache_key = md5( maybe_serialize( $args ) );
		$cached_queries = wp_cache_get( 'app_get_appointments' );
		if ( ! is_array( $cached_queries ) ) {
			$cached_queries = array();
		}

		if ( isset( $cached_queries[ $cache_key ] ) ) {
			$results = $cached_queries[ $cache_key ];
		}
		else {
			$where = array();

			if ( false !== $args['worker'] && $worker = appointments_get_worker( $args['worker'] ) ) {
				$where[] = $wpdb->prepare( "worker = %d", $worker->ID );
			}

			if ( false !== $args['location'] ) {
				$where[] = $wpdb->prepare( "location = %d", $args['location'] );
			}

			if ( false !== $args['user'] ) {
				$where[] = $wpdb->prepare( "user = %d", $args['user'] );
			}

			if ( false !== $args['service'] && ! is_array( $args['service'] ) ) {
				// Only one service, let's make it an array
				$args['service'] = array( $args['service'] );
			}

			if ( ! empty( $args['service'] ) && is_array( $args['service'] ) ) {
				$where_services = array();
				foreach ( $args['service'] as $service_id ) {
					$service = appointments_get_service( $service_id );
					if ( ! $service ) {
						continue;
					}

					$where_services[] = absint( $service_id );
				}
				if ( ! empty( $where_services ) ) {
					$where[] = 'service IN (' . implode( ',', $where_services ) . ')';
				}
			}

			if ( ! empty( $args['date_query'] ) && is_array( $args['date_query'] ) ) {
				$date_query_where = array();
				$date_queries = $args['date_query'];

				// Set the date queries conditions
				$allowed_conditions = array( 'AND', 'OR' );
				if ( ! isset( $args['date_query']['condition'] ) ) {
					$condition = 'AND';
				}
				else {
					$condition = strtoupper( $args['date_query']['condition'] );
				}

				if ( ! in_array( $condition, $allowed_conditions ) ) {
					$condition = 'AND';
				}

				// Parse every Date query
				foreach ( $date_queries as $key => $date_query ) {
					if ( 'condition' === $key ) {
						continue;
					}
					$date_query = _appointments_parse_date_query( $date_query );
					if ( $date_query ) {
						$date_query_where[] = $wpdb->prepare( $date_query['field'] . $date_query['compare'] . "%s", $date_query['value'] );
					}
				}

				if ( $date_query_where ) {
					$where[] = '(' . implode( " " . $condition . " ", $date_query_where ) . ')';
				}
			}

			if ( ! empty( $args['app_id'] ) && is_array( $args['app_id'] ) ) {
				$args['app_id'] = array_map( 'absint', $args['app_id'] );
				$where[] = 'ID IN ( ' . implode( ',', $args['app_id'] ) .  ' )';
			}

			if ( $args['status'] ) {
				$statuses = array();
				if ( is_array( $args['status'] ) ) {
					foreach ( $args['status'] as $status ) {
						if ( array_key_exists( $status, appointments_get_statuses() ) ) {
							$statuses[] = $status;
						}
					}
				}
				elseif ( is_string( $args['status'] ) && array_key_exists( $args['status'], appointments_get_statuses() ) ) {
					$statuses = array( $args['status'] );
				}

				$where[] = 'status IN ("' . implode( '","', $statuses ) . '")';
			}

			$first = date( 'Y-m-d 00:00:00', strtotime( $args['date'] ) );
			$first_timestamp = strtotime( $first );
			$last_timestamp = strtotime('+1 day', $first_timestamp);		
			$last = date( 'Y-m-d H:i:s', $last_timestamp );

			$where[] = $wpdb->prepare( "start >= %s", $first );
			$where[] = $wpdb->prepare( "end < %s", $last );

			if ( false !== $args['s'] ) {
				$args['s'] = trim( $args['s'] );
				if ( $args['s'] ) {
					// Search by user name
					$where[] = $wpdb->prepare(
						"( name LIKE %s OR email LIKE %s OR user IN ( SELECT ID FROM $wpdb->users WHERE user_login LIKE %s OR user_nicename LIKE %s OR display_name LIKE %s OR user_email LIKE %s ) )",
						'%' . $args['s'] . '%',
						'%' . $args['s'] . '%',
						'%' . $args['s'] . '%',
						'%' . $args['s'] . '%',
						'%' . $args['s'] . '%',
						'%' . $args['s'] . '%'
					);
				}
			}

			if ( ! empty( $where ) ) {
				$where = "WHERE " . implode( " AND ", $where );
			}
			else {
				$where = '';
			}

			$allowed_orderby = array( 'ID', 'created', 'user', 'name', 'email', 'location',
				'service', 'worker', 'price', 'status', 'start', 'end' );
			$allowed_order = array( 'DESC', 'ASC', '' );
			$order_query = '';
			$args['order'] = strtoupper( $args['order'] );
			if ( in_array( $args['orderby'], $allowed_orderby ) && in_array( $args['order'], $allowed_order ) ) {
				$orderby = $args['orderby'];
				$order = $args['order'];
				$order_query = "ORDER BY $orderby $order";
			}

			$limit = '';
			if ( $args['per_page'] > 0 ) {
				$limit = $wpdb->prepare( "LIMIT %d, %d", intval( ( $args['page'] - 1 ) * $args['per_page'] ), intval( $args['per_page'] ) );
			}

			$found_rows = '';
			if ( $args['count'] ) {
				$found_rows = 'SQL_CALC_FOUND_ROWS';
			}

			$query = "SELECT $found_rows * FROM $table $where $order_query $limit";
			$results = $wpdb->get_results( $query );

			if ( $args['count'] ) {
				$results = $wpdb->get_var( "SELECT FOUND_ROWS()" );
			}

			if ( $results && ! $args['count'] ) {
				$cached_queries[ $cache_key ] = $results;
				wp_cache_set( 'app_get_appointments', $cached_queries );
			}
		}

		if ( ! $args['count'] ) {
			$apps = array();
			foreach ( $results as $row ) {
				wp_cache_add( $row->ID, $row, 'app_appointments' );
				$apps[] = new Appointments_Appointment( $row );
			}

			return $apps;
		}

		return $results;

	}

}
?>

Can be called like:

$args = array(
	'per_page' 		=> 50,
	'page' 			=> 1,
	'status' 		=> array(
						'confirmed',
						'paid'
					),
	'orderby' 		=> 'start',
	'order' 		=> 'ASC',
	'date'			=> '2017-01-28'
);

$apps = appointments_get_appointments_for_day( $args );