Continuity With Meditation Practice
I hate to sound too critical, because you appear to have a healthy meditation regime, so kudos for that. Having said that, adhering too strictly to a specific routine runs the risk of being silabbatupadana, i.e. clinging to a ritual for it's own sake.

Based on contemporary learning theory, more frequent meditation is likely more favorable for developing the dhyanas, over a strict routine, or having few session with longer duration, for instance. I wouldn't be surprised if the suttas reflects these findings.

To sum up: a scheduled routine helps, as well as meditating often, even when it's outside your regular routine.