Lucky Seven

The number seven, as a source of luck.

Examples of Lucky Seven include:

Anime and Manga

The seventh volume of the Lucky Star manga has a tiny "Lucky" beside the volume number.

Subverted in Unstoppable: Train #777 (888 in Real Life) is loaded with hazardous chemicals and speeding towards a populated area.
Referenced in the title of Lucky Number Slevin, which is a pun on the phrase and Slevin, the main character's name.

Survivor season seven winner Sandra Diaz-Twine became the first (and, so far, only) two time winner by winning season twenty, seven years after her first win.

Harry Potter - seven books; seven years at Hogwarts; six Horcruxes, so his soul is split into seven (or, Voldemort thinks that his soul is in seven; actually, his soul is split into eight, and this last Horcrux proves to be his downfall). According to Word of God, seven is a lucky number in the Potterverse.

Machine Robo, or Go Bots: Four figures marked #7 are all Gaurdians (Friendly Gobots).
- Turbo = MR 7
- Baron Von Joy = DX Scale Robo #7
- Rest Q (Anime Reissue) = MRB 7
- Leader 1 (Also anime reissue) = MRJ 7.
Transformers probably hates this trope:
- Bluestreak (later Silverstreak) was Diaclone Car Robo #7, but was not so popular as Prowl (Car Robo #13).
- On the Microman side, Micro Change #7 was designated as one of the evil Decepticons!

All over the place with Punch-Out!! Wii's version of Aran Ryan. He always gets up from a knockdown on the seventh count, you can hit him seven times when he's stunned, and sevens pop up in his challenges in Exhibition Mode. He's Irish, so it's a "lucky seven"/"luck of the Irish" gag.
Final Fantasy:
- Final Fantasy VII (figures) causes a character to go berserk if their HP hits 7777, attacking all enemies up to 63 times for 7777 damage per hit: the only enemies that can withstand this and not die are the Bonus Bosses. The Lucky Seven concept continues in the prequel Crisis Core, where getting three Sevens on the DMV (the battle system's slot-machine gimmick) will level up your character.
- Final Fantasy IX contains the attack "Lucky 7s", a Shout-Out to the above.
- In Final Fantasy VI, Setzer has a slot-machine-like attack, and getting three 7's kills all enemies.
The seventh Touhou game is the only one to feature the "Supernatural Border" system, which can and will save your life on occasion. It is also the first game in the series to have visible hitboxes.
Mario Kart 7 features an item called "Lucky 7", which grants the player seven items at one time.

The Seven Lucky Gods of Japanese mythology.
Inverted with the concept of the Seven Deadly Sins. And played straight with the opposing concept of the Seven Heavenly Virtues.
In Judeo-Christian numerology, Seven is considered the number of God.

As an homage to this trope, the highest jackpots on a slot machine is often assigned to three 7's (which, perhaps not coincidentally, add up to the ideal number in blackjack).
Seven is the most likely roll of a pair of standard 6-sided dice. In craps, you can win by rolling a seven (or eleven) first, and lose by getting seven on any subsequent roll. The fact that seven is the most likely sum for the outcome of rolling two dice (with a probability of 1/6) is likely the origin of the whole "seven is a lucky number" superstition.
Zayin is the 7th letter in most Semitic abjads (writing system). It is the source of both the modern "7" and "Z", and also meant "sword" (several proto-versions look like basic swords, and the Arabic derivation bears a slight resemblance to a scimitar).