This piece is a theatrical song reduction from the early 1900s. Its musical identity is defined by clean, diatonic harmony and a structured, predictable melodic development.
The primary challenge is to balance the texture so the melodic line projects clearly over the accompaniment.
Comparable difficulty and character — good candidates for what to learn next.
'Twas in September is rated Henle level 2 of 9 — beginner repertoire, roughly ABRSM grades 2–3. In the RCM system it corresponds to about level 4.
You should be comfortable at Henle level 2 — meaning you can already play level 1–2 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 'Twas in September lasts about 3 minutes.
'Twas in September is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include At Twilight by Rudolf Friml, Bijou by Chiquinha Gonzaga, Au Hameau (In the Hamlet) by Franz Behr — see the full similar-pieces list on this page.