The event that prompts the most thank you notes to be written is your wedding, but I believe everyone should write thank you notes for gifts they receive, whether it be a birthday, Valentine's day, graduation, etc.
For weddings, you have 3 months from the day you receive a gift to write a thank you note. I suggest writing the note ASAP because according to all the etiquette authorities, you have 3 months after the wedding to finish all the rest of your thank you notes from the gifts you get from the wedding. (But remember, people have until your first anniversary to send you a wedding present before it's a faus pax!)
For non-wedding related gifts, etiquette dictates a 2-week turn around before the thank you note goes stale, so it’s good to try writing one within a week, giving it another few days to travel through the USPS.
If the gift is for you and your significant other, whoever is closer to the gifter writes the letter. Use a quality black pen (I recommend the Pilot G2 Mini Pen, because "splotchy ink does not a thank you note make." - Miss Manners.) Also, write the note in the first person, but be sure to include the spouse’s name in the letter. (For example, “Kate and I greatly appreciate the candle sticks…”)
A thank you note should include the following sentences, in this order:
Begin the note with the gifter’s informal names, “Dear Janice and Bill” unless the gifter is an acquaintance of your parents, in which case use honoraries, “Dear Dr. & Mrs. Finegold..."
Thank them for the gift. Be sure to name the gift in detail! Don’t just say “Thank you for the platter” instead write, “Thank you for the beautiful china platter with the sunflower pattern!” Include the quantity if it’s multiple gifts, "Thank you for the four nambe napkin-holders.” If it’s multiple gifts, list them. If the gift is cash, it’s up to the writer’s discretion to include the dollar amount or not. If it’s a donation in your honor, thank the gifter graciously for their contribution to this important cause.
Discuss Use. Even if you hate it and return it, the people who bought it for you want to hear that you love it!! For example, “These day-glow plaid snowman sweaters are perfect for the chilly weather in Massachusetts.”It is crucial to include a desired purchase with the money given, “Barry and I are putting the gift towards the purchase of our new couches." If it’s a donation to a charity, mention how that charity will benefit the community.
Mention the Past, Allude to the Future. If the thank you is sent before the wedding, acknowledge if they will be attending or not, “I’m sorry to hear that you can’t make it to the wedding…” If it’s after the fact, acknowledge their presence! “It was so wonderful seeing you and the kids at the wedding!” The same goes for graduation gifts, birthdays, etc. If they came, mention it, if they can’t come, suggest seeing each other soon.
If you’re close, personalize it! Include a sentence or two from the heart. "I can't believe you and John started dating thanks to my wedding..." Otherwise, say something nice: “It meant so much to my family that you flew up from DC…”
Closing the letter is two fold. First Grace: “Thanks again for your gift,” followed by Regards “From us both,” (“Love from us Both,” “The Best from us Both,” etc.) and then the writer’s name. Thank you cards are written by one person and should be signed by that person only.
Here's what NOT to include:
No news about your life. This is not a time to update them about future plans, vacations, etc. - No photos. That is a separate letter with a separate agenda! - If the gifter is in for another thank you note for multiple gifts (say, if your birthday falls near a gift-giving holiday), make sure to keep the notes separate. Don’t include a wedding-shower gift along with the birthday thank you note. Send a separate thank you note attached for each.