In this act, you write poems for Sayori, Yuri, or Natsuki. You talk to each of them and spend time with your chosen lover; Sayori usually likes your poems regardless of who you wrote them for, but comments later that she knows you wrote them for Natsuki or Yuri. While she tries to hide her sorrow, it is apparent that losing your attention bothers Sayori. If you check the game files, you'll notice that Monika puts files or documents in the game files after you have written every single poem, regardless of for whom it was written.
Sayori will be the only person who confesses to you in Act 1. While rejecting her confession results in heartbreak, accepting her confession does not alter the path of the game, save for a few lines of dialogue and a CG. Despite the fact that on the surface the game doesn't appear to have been altered yet, Sayori's personality has seen a fundamental change over the course of Act 1, from a happy, bubbly personality to a more melancholy, bittersweet one (and from depressed to all-out suicidal in the span of just a few days), in which Monika's tampering with her depression is heavily implied (and later explicitly stated) to have played a part. Act 1 will end when the player sees Sayori's body hanging in her room.
If the player chooses to write for Natsuki, the second day will have them find her searching for her manga in the classroom closet. The player will then proceed to spend the club time with her, reading the manga, "Parfait Girls", which Natsuki particularly likes. The player will receive a CG of them reading the manga.
Once the time to share poems arrives, Monika, Sayori, and Yuri comment on the likeness to Natsuki's writing style, whereas Natsuki is flustered and dismisses the player's poem as merely "okay," showing traits typical of the tsundere type to which she is likened.
If the player chooses to write for Yuri, the second day will entail reading a book which she had bought for them (although she denies doing this), which shows a CG of reading beside each other from the same book.
When sharing the player's poem with Yuri, she is taken aback but provides constructive feedback for the player to consider and use to improve their writing style.
On day three, Monika is found to have moved all of Natsuki's manga up to the top shelf, and Natsuki is forced to find a way to reach them. This awards the player with a CG as the player holds onto the chair while Natsuki grabs the box of manga. Once she has the box, she asks the protagonist to take it, but she changes her mind as she realizes the protagonist can see up her skirt. Her anger in this situation causes her to fall, and she lands on top of the protagonist, accidentally causing him to crease a page in one of the manga.
On the third day, Yuri and the protagonist continue where they left off in the book from the second day. Yuri requests to sit against the wall on the floor, rather than at the desks, due to her frequent back pain. The protagonist suggests this is due to her bad reading posture, which Yuri goes along with in embarrassment. Yuri prepares tea, and the protagonist takes out chocolates. Yuri expresses how she can't hold chocolate while holding her book, prompting the protagonist to feed her them, earning the player a CG in the process.