Imaizumi Brings All the Gals to His House is a series with three different content streams that are part of the same story, and how they fit together isn't immediately obvious. To that end, I present:
The Grand Unified Imaizumi timeline
D1: Reina begins the physical relationship
M1: references the events of D1, with two flashbacks that set up D1.
P(12,15-16, 17, 19-20, 21) Eguchi storyline: must occur before M2
P32: Tsutsumin tasks Eguchi with ensuring Ruri sits next to Keita during the seat change.
M2: Ruri ends up sitting next to Keita in this chapter, due to Eguchi in P32.
M7: Keita begins physical fitness training
P7: Keita eating heartily, he mentions eating more as part of his training plan.
D2: opens with Reina coaching Keita with weighted push-ups, implying he's begun physical training prior.
P40: upcoming basketball match mentioned, specifically with Tsutsumin as a secret weapon.
M8+9: Basketball match occurs.
M10+11: Imaizumi's mom shows up at the apartment.
P11: Reina asks Lisa to cover for her and also Yukina, essentially spilling the beans.
P27: Club Pres Kaho makes Ruri the cheerleader costume she wears in early D3.
D3: Lisa shows up at the old apartment
M14: move to the new apartment begins
D4: One month after D3 ending, apartment move completed. Yukina asks Keita to call her by her first name.
M15-16: simultaneous with events of D4
P20.1: must be after D4/M16, also during summer
M17: Opens with a flashback to D4.
M20+21: occurs one month after M10+11, per Imaizumi's mom.
P37: Ruri tries on a bunny suit Kaho has made.
M23: girls in said bunny suits.
P16.1: Halloween, must be before M26/D5
D5: timeskip from late Dec to early April beginning 2nd year of high school.
M27: directly after returning from the trip in D5, pre-timeskip.
M28: New Years Day and Hatsumode, again pre-timeskip.
P5: on or around Valentine's day. Since the school year begins in April and this couldn't have occurred before they were in high school, it must occur in Feb during the D5 time skip.
D6: Presumably the latest point on the timeline currently.
Not yet placed concretely in the timeline
P6: 3 months after D1, as Keita hasn't jerked off since then presumably.
P28: mom visit, time unknown
P31: probably occurs before M31, maybe the first naked apron?
Hopefully this makes things easier to follow!