Warning: Contains SPOILERS for The Handmaid's Tale season 6, episode 8!
The red dresses are the most iconic piece of iconography from The Handmaid's Tales season 6, episode 8 uses them in a bigger way than ever before, with this June and Mayday storyline giving them a new thematic significance.
The Handmaid's Tale season 6 has followed June as she works with Mayday to take down Gilead. At first, they wanted to attack Jezebel's, wiping out the Commanders who frequent the brothel. Unfortunately, Nick told Commander Wharton about the attack in The Handmaid's Tale season 6, episode 6. Thus, June had to come up with a new plan in episode 7. She decided to attack the wedding of Serena Joy and Commander Wharton, knowing that this would be a hot spot for Commanders. So, The Handmaid's Tale season 6, episode 8 sees this attack finally get carried out.
What The Handmaids' Red Dress Originally Meant
It Was A Symbol Of Gilead's Oppression
Throughout The Handmaid's Tale, the red dresses have always been a symbol of oppression. The long, flowing robes are meant to hide the bodies of the women who wear them, with this showing the sexism of the fascist theocracy. They also strip the Handmaids of any individuality, with each of them looking exactly like all of the others. The red of the Handmaids' dresses is a symbolic reflection of fertility and menstrual blood, showing that they aren't seen as anything more than their ability to produce children.

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June hits all of these points during her speech at the beginning of The Handmaid's Tale season 6, episode 8. June talks about how Gilead has used clothing to oppress women, highlighting how the red dresses were essential in doing this. However, June wants the Handmaids to reclaim the red dresses.
How The Handmaid's Tale Season 6 Gives The Dress A New Meaning
It Turns It Into A Symbol Of Revolution
June continues the speech by explaining how, with the attack on Serena Joy's wedding, the Handmaids will be giving the red dresses a new meaning. Although Gilead has always used the dresses as a weapon against women, the Handmaids are now using the red dresses as a weapon against Gilead. The dresses now act as a team uniform, making the Handmaids a fearsome army.
The long, flowing robes that were once meant to hide their bodies now allow them to conceal knives that are used to kill the Commanders. The anonymity caused by the dresses now allows the Handmaids to blend in with one another, making each soldier in June's army indistinguishable from the crowd. The red of the dresses now strikes fear into the hearts of the Commanders who see it, with the red symbolizing the blood of the oppressors. The Handmaids have reclaimed the symbolism of the red dresses, giving them an all-new meaning.
Changing The Red Dress Makes Sense Of The Handmaid's Tale's Ending
It Brings Things Full Circle
Allowing June to change the meaning of the red dresses makes sense, with it bringing things full circle for The Handmaid's Tale's ending. Based on the existence of the sequel series The Testaments, Gilead probably isn't going to fall by the end of season 6. Thus, June will fail to liberate the women of Gilead, meaning that the Handmaid system will most likely still exist.

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However, the red dress is now a symbol of revolution within Gilead, reminding the Handmaids to keep fighting. While Gilead may be around for a while longer, it will eventually fall. The red dresses are a reminder of this, carrying on the legacy of June and those who fought against Gilead throughout The Handmaid's Tale.