I'm going to cheat on this one a little bit, because I can't really choose one over the other. As such, here I tack on a plural.
Day 7 - Your Best Friends
Both my best friends are female, and we've all been "family" for at least five or six years, almost certainly longer.
One's quite possibly the most blunt and obstinate girls I've ever known. She doesn't dig herself into holes, she digs herself into foxholes, in which she stands her ground. If she feels she's right and I'm wrong about something, she's perfectly fine with ripping me a new one, and if I defend myself successfully (or if the reverse happens), she will concede. She's big on modern medicine and technology, is capable of building her own computers and doing basic programming, and is about as close to a straight-up capitalist as you'll ever find. Her naming scheme for computers comes from the X-Men.
The other one is significantly softer, but exhibits similar qualities in terms of personality. However, in terms of belief, she couldn't be more diametrically opposite: she leans heavily towards the Granola Girl stereotype, with firm belief in homeo- and naturapathy, alternative medicines, an inherent dislike but resigned acceptance of all the radio waves floating through the air and our heads, and a tendency towards organics. Her naming scheme for computers (and portable devices, and hard drive partitions) stems from Disney characters and Winnie the Pooh.
Both of them are Trekkies, and although not avowed Browncoats, they enjoyed Firefly quite a bit (the latter is a Whedonite, the former sci-fi in general). For years, they lived in the same apartment complex (one floor away from one another, on opposite ends of the staircase), because one helped the other into it. This meant weekends of us all hanging out together watching different television shows and movies, cooking food, and cracking wise.
The three of us? Inseparable. Even with marriages and now a kid in the former's case, boyfriends and choirs in the latter's case, and a terminal case of I'm-busy-with-something in mine, we all find time to hang out with one another--if not all together, in massive amounts of one-on-one.
Day 01 - Introduce yourself
Day 02 - Your first love
Day 03 - Your parents
Day 04 - What you ate today
Day 05 - Your definition of love
Day 06 - Your day
Day 07 - Your best friend
Day 08 - A moment
Day 09 - Your beliefs
Day 10 - What you wore today
Day 11 - Your siblings
Day 12 - What’s in your bag
Day 13 - This week
Day 14 - What you wore today
Day 15 - Your dreams
Day 16 - Your first kiss
Day 17 - Your favorite memory
Day 18 - Your favorite birthday
Day 19 - Something you regret
Day 20 - This month
Day 21 - Another moment
Day 22 - Something that upsets you
Day 23 - Something that makes you feel better
Day 24 - Something that makes you cry
Day 25 - A first
Day 26 - Your fears
Day 27 - Your favorite place
Day 28 - Something that you miss
Day 29 - Your aspirations
Day 30 - One last moment