Abstract Protostars likely accrete material at a highly time-variable rate, but measurements of accretion variability from the youngest protostars are rare, as they are still deeply embedded within their envelopes. Submillimeter/millimeter observations can trace the thermal response of dust in the envelope to accretion luminosity changes, allowing variations in the accretion rate to be quantified. In this paper, we present contemporaneous submillimeter/millimeter light curves of variable protostars in Serpens Main, as observed by the ALMA Atacama Compact Array (ACA), the Submillimeter Array (SMA), and the James Clerk Maxwell Telescope (JCMT). The most recent outburst of EC 53 (V371 Ser), an ∼18 month periodic variable, is well sampled in the SMA and JCMT observations. The SMA light curve of EC 53 is observed to peak weeks earlier and exhibit a stronger amplitude than at the JCMT. Stochastic variations in the ACA observations are detected for SMM 10 IR, with an amplitude a factor of ∼2 greater than that seen by the JCMT. We develop a toy model of the envelope response to accretion outbursts to show that EC 53's light curves are plausibly explained by the delay associated with the light travel time across the envelope and the additional dilution of the JCMT response, due to the incorporation of cold envelope material in the beam. The larger JCMT beam can also wash out the response to rapid variations, which may be occurring for SMM 10 IR. Our work thus provides a valuable proof of concept for the use of submillimeter/millimeter observations as a means of probing both the underlying accretion luminosity variations and the protostellar environment.