It's been awhile since I worked on this, so some further thoughts
Fall Weather Friends
September 20-21 2 days w The Running of the Leaves on September 21. IMO, this is the start of Autumn & is 3 months after The Summer Sun Celebration
Ciderfests are usually in October, so first weekend in October.
Best Night Ever.
November 21, year 1. 1 or 2 days (depending when you think the party broke up)
It's the 21st of some month (Ticket Master). The S5 Episode Make New Friends....Discord must be at least November. year 2 or in Spring.
May the Best Pet Win
Spring, year 1. Tanks For the Memories (S5) is Winter, year 2 & it's Dash's first Winter w Tank
Hearts & Hooves Day
Pony Valentine Day (Feb. 14)? This is a problem because it's picnic weather (probably Spring) & February is mid Winter. This means either
1) Winter is over no later than, say, Feb 10 (to allow for Winter Wrap Up). This means that Winter is very short. This makes sense because it explains "Why so few Winter episodes?" It's because "There isn't much Winter to have adventures IN"
The problem is that Rarity specifies"90 days of Winter" (song "Winter Wrap Up") So either this is a continuity error or Winter starts in early November, shortly after Nightmare Night. This makes Autumn or Autumn starts in August.
2) IMO, H&HD has to be MUCH later. Somewhere around Easter to around Mother's Day (early May)
June, Year 2
One Bad Apple
School seems to be out. They mention a Summer Harvest Festival. The calendar says otherwise, but in the USA Summer is considered to run from Memorial Day weekend (last Monday in May) to Labor Day weekend (first Monday in September). So, June, year 2.
Secret of My Excess
Spike is hatched during Twilight's school application. (Cutie Mark Chronicles) In the USA, this would be between semesters. So, early May to the end of July. I'd say July, year 1 except the Crusaders have a cameo (Spike steals Scootaloo's scooter). Either this is a continuity error or the episode is after Call of the Cutie. So, it's late May year 1 or early June, year 2
It seems to be a long time since Boast Busters, so I'm putting it in June.
IMO, one of these 3 episodes is almost certainly the last episode before Magical Mystery Cure.
Magical Mystery Cure
Has to end no later than June 18, year 2. If you accept EQ Girls as canon, then no later than June 13