Possible Clues:

• Insta hashtag for nostalgic content
• Insta hashtag whose middle letter stands for "back"
• Palindromic hashtag whose last letter stands for "Thursday"
• Hashtag for retro pics
• "Blast from the past" hashtag
• Hashtag for a blast-from-the-past pic
• Instagram hashtag accompanying a nostalgic photo
• Hashtag for some nostalgic Insta photos
• #thisphotoisamemory