In the following question, out of the four alternatives, select the alternative which best expresses the meaning of the idiom or phrase “Red letter day”.

Introduction / Context:
This question focuses on the idiom red letter day, a phrase that has its roots in older calendars where important festivals and saints days were marked in red ink. Today it is still used in spoken and written English to describe days that are especially important or memorable. Your task is to choose the option that best captures this figurative meaning.

Given Data / Assumptions:
- Idiom: Red letter day. - Options: Starting day, Holiday, Significant day, Ending day. - The idiom describes the character of a particular day.

Concept / Approach:
Historically, red letters in calendars marked special dates that stood out from ordinary days, such as religious festivals or major events. Over time, red letter day has come to mean any day that is special, important or highly memorable in a positive sense. Therefore, the correct explanation among the options is the one that generalises this idea as a significant day, not necessarily the first or last day of something, and not just any holiday.

Step-by-Step Solution:
Step 1: Recall the general meaning: a red letter day is one that stands out in memory because something very good or important happens. Step 2: Examine Significant day. A significant day is an important, notable or memorable day, which directly matches the idiom. Step 3: Examine Starting day. This simply means the first day of an activity or event and is not always memorable in a special way. Step 4: Examine Holiday. While some red letter days may be holidays, not every holiday is especially significant or personally memorable, and not every significant day is a public holiday. Step 5: Examine Ending day. This is the last day of some period or event and does not necessarily have special positive significance.

Verification / Alternative check:
Use the idiom in a sentence: The day I received my first job offer was a real red letter day in my life. Rewriting this sentence with significant day gives The day I received my first job offer was a significant day in my life, which preserves the meaning. Replacing red letter day with holiday or starting day would not be accurate, because the day might not be a public holiday or the beginning of anything. This shows that significant day is the correct interpretation.

Why Other Options Are Wrong:
Starting day is wrong because the idiom has nothing to do with the first day of an event; it is about importance and memorability. Holiday is wrong because the phrase red letter day emphasises special significance rather than legal or public holiday status. Ending day is wrong because finishing something does not automatically make that day special or joyous.

Common Pitfalls:
A common mistake is to link red letter day with calendar holidays only, because of its historical origin. However, modern usage is much broader and includes any personally or historically important day, such as a wedding day, graduation day or victory day, whether or not it is a public holiday. To avoid confusion, remember that the essence of the idiom is importance and happy memorability, which is well expressed by the phrase significant day.

Final Answer:
“Red letter day” means a Significant day, that is, a day of special importance or happy memory.