The takeaway
Vanguard Short-Term Treasury Index Fund ETF Shares shows a slight seasonal lean over 10 years of data — strongest in March (+0.3%) and softest in September (−0.1%).
Right now
In July, the fund has risen 80% of years, averaging +0.2%, roughly 1.9 pts behind the S&P 500.
The full picture
Vanguard Short-Term Treasury Index Fund ETF Shares's most dependable month has been March, higher in 8 of 10 years; September has been its least reliable, up just 30% of the time.
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Month by month
Across the year the fund has stayed close to the S&P 500 — no single month stands out as a real edge.
“vs S&P” is Vanguard Short-Term Treasury Index Fund ETF Shares’s average for a month minus the S&P 500’s average for that same month — isolating Vanguard Short-Term Treasury Index Fund ETF Shares’s own seasonal edge from broad market drift.
Reality check
Over the last 5 years, March has closed higher 60% of the time versus 80% across the last 10 years — the pattern is weakening.
Figures are the typical (median) March return and how often it rose — the last 5 years versus the last 10(the heatmap’s default window). This verdict stays anchored to that 10-year window even if you zoom the chart, so it never disagrees with the badges above.
In plain English
This is a fund you can almost set a calendar by, and March is the anchor — it has closed higher in 8 of 10 Marches, the steadiest beat on its year.
The strength looks broad-based rather than freakish: its average (+0.3%) and median (+0.2%) sit close together, so no single blow-out year is flattering the figure. Few peers keep such company in March — the typical stock clears it just 56% of the time.
March anchors a run, too: the March-through-August window has been the fund's reliable season. At the other end of the calendar, September is the year's quietest corner, essentially flat on average, and the edge isn't year-round — the fund has trailed the S&P 500 in November, July, and April.
The pattern has softened of late, March's last five years slipping below its longer-run record.
For a fund this dependable in March, the sharper question is the rest of the year — outside its strong stretch, the calendar gives far less to lean on.
Short answers on the fund's best month (March), its worst (September), and whether it really trades seasonally.
Only mildly. The fund's months are fairly even — March is the firmest (+0.3%) and September the softest (−0.1%), a narrow spread that points to weak seasonality rather than a strong calendar effect.
March has been the strongest, averaging +0.3% and closing higher in 8 of 10 years since 2016.
It's the weakest, averaging −0.1% — historically a soft spot, though it still varies from year to year.
Explore
These names have the strongest July track records on record — a starting point for comparison.
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