Merry and Mary are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Merry is an adjective. It means happy or delighted. For whatever reason, the only time we hear “Merry” a lot it is at Christmastime. Happy is a far more frequently used words. Merriment is a related noun form of Merry.
Mary is a noun. It’s one of the oldest female names in the western world. It was the name of several biblical figures, including Jesus’ mother. It’s also a name frequently used in English, French, and Spanish royalty.
The following story uses both words correctly:
Mary really disliked the month of December because everyone said “Merry Christmas” all the time. It was very distracting to hear what sounded like her name constantly. When she moved to a country with different winter greetings, she was much happier, not to mention more productive.