This matters more in 2026 than in an ordinary year if your property sits in an area affected by the province's current reassessment cycle, since a changed assessed value triggers its own notice and its own deadline, separate from any earlier notice you may have already dealt with.
The safest approach is to treat every notice that arrives in 2026 as carrying its own clock, rather than assuming it matches a deadline from a previous year or from a neighbour's notice, since mailing dates are not identical across every property.
Read the date on your own notice before making any decision about filing. Get your hand-reviewed valuation from Joel Dyck.