This is an example of a functions.php file with custom text added. It tries to be comprehensive and address all translation methods (contextual and singular/plural)

sample function with changed text.php

<?php

/*
* EXAMPLE OF CHANGING ANY TEXT (STRING) IN THE EVENTS CALENDAR
* See the codex to learn more about WP text domains:
* http://codex.wordpress.org/Translating_WordPress#Localization_Technology
* Example Tribe domains: 'tribe-events-calendar', 'tribe-events-calendar-pro'...
*/
function tribe_custom_theme_text ( $translation, $text, $domain ) {
	// If this text domain doesn't start with "tribe-", "the-events-", or "event-" bail.
	if( (strpos($domain, 'tribe-') !== 0 || strpos($domain, 'the-events-') !== 0 || strpos($domain, 'event-') !== 0) ) {
		return $translation;
	}

	// Put your custom text here in a key => value pair
	// Example: 'Text you want to change' => 'This is what it will be changed to'
	// The text you want to change is the key, and it is case-sensitive
	// The text you want to change it to is the value
	// You can freely add or remove key => values, but make sure to separate them with a comma
	// This example changes the label "Venue" to "Location", and "Related Events" to "Similar Events"
	$custom_text = array(
		'Venue' => 'Location',
		'Related %s' => 'Similar %s',
	);

	// If we don't have replacement text in our array, return the original (translated) text.
	if ( empty( $custom_text[$translation] ) ) {
		return $translation;
	}

	return $custom_text[$translation];
}

function tribe_custom_theme_text_plurals ( $translation, $single, $plural, $number, $domain ) {
	// If this text domain doesn't start with "tribe-", "the-events-", or "event-" bail.
	if( (strpos($domain, 'tribe-') !== 0 || strpos($domain, 'the-events-') !== 0 || strpos($domain, 'event-') !== 0) ) {
		return $translation;
	}

	/** If you want to use the number in your logic, this is where you'd do it.
	 * Make sure you return as part of this, so you don't call the function at the end and undo your changes!
	 */

	// If we're not doing any logic up above, just make sure your desired changes are in the $custom_text array above (in the `tribe_custom_theme_text` filter. )
	if ( 1 === $number ) {
		return tribe_custom_theme_text ( $translation, $single, $domain );
	} else {
		return tribe_custom_theme_text ( $translation, $plural, $domain );
	}
}

function tribe_custom_theme_text_with_context ( $translation, $text, $context, $domain ) {
	// If this text domain doesn't start with "tribe-", "the-events-", or "event-" bail.
	if( (strpos($domain, 'tribe-') !== 0 || strpos($domain, 'the-events-') !== 0 || strpos($domain, 'event-') !== 0) ) {
		return $translation;
	}

	/** If you want to use the context in your logic, this is where you'd do it.
	 * Make sure you return as part of this, so you don't call the function at the end and undo your changes!
	 * Example (here, we don't want to do anything when the context is "edit", but if it's "view" we want to change it to "Tribe"):
	 * if ( 'edit' === strtolower( $context ) ) {
	 * 		return $translation;
	 * } elseif( 'view' === strtolower( $context ) ) {
	 *  	return "Tribe";
	 * }
	 *
	 * Feel free to use the same logic we use in tribe_custom_theme_text() above for key => value pairs for this logic.
	 */

	// If we're not doing any logic up above, just make sure your desired changes are in the $custom_text array above (in the `tribe_custom_theme_text` filter. )
	return tribe_custom_theme_text ( $translation, $text, $domain );
}

function tribe_custom_theme_text_plurals_with_context ( $translation, $single, $plural, $number, $context, $domain ) {
	// If this text domain doesn't start with "tribe-", "the-events-", or "event-" bail.
	if( (strpos($domain, 'tribe-') !== 0 || strpos($domain, 'the-events-') !== 0 || strpos($domain, 'event-') !== 0) ) {
		return $translation;
	}

	/** If you want to use the context in your logic, this is where you'd do it.
	 * Make sure you return as part of this, so you don't call the function at the end and undo your changes!
	 * Example (here, we don't want to do anything when the context is "edit", but if it's "view" we want to change it to "Tribe"):
	 * if ( 'edit' === strtolower( $context ) ) {
	 * 		return $translation;
	 * } elseif( 'view' === strtolower( $context ) ) {
	 *  	return "cat";
	 * }
	 *
	 * You'd do something as well here for singular/plural. This could get complicated quickly if it has to interact with context as well.
	 * Example:
	 * if ( 1 === $number ) {
	 * 		return "cat";
	 * } else {
	 *  	return "cats";
	 * }
	 * Feel free to use the same logic we use in tribe_custom_theme_text() above for key => value pairs for this logic.
	 */

	// If we're not doing any logic up above, just make sure your desired changes are in the $custom_text array above (in the `tribe_custom_theme_text` filter. )
	if ( 1 === $number ) {
		return tribe_custom_theme_text ( $translation, $single, $domain );
	} else {
		return tribe_custom_theme_text ( $translation, $plural, $domain );
	}
}

// base
add_filter('gettext', 'tribe_custom_theme_text', 20, 3);
// Plural-aware translations
add_filter('ngettext', 'tribe_custom_theme_text_plurals', 20, 5);
// translations with context
add_filter('gettext_with_context', 'tribe_custom_theme_text_with_context', 20, 4);
// Plural-aware translations with context
add_filter('ngettext_with_context', 'tribe_custom_theme_text_plurals_with_context', 20, 6);