File when the qualifying year is complete and evidenced, and when the US site can genuinely offer the managerial role described. Filing during the off-season, before the transferee is needed, gives room for a request for further evidence without forcing a start date that the decision has not yet permitted.
Let the evidence, not the season, set the filing date
There is a temptation in a seasonal business to file in the spring so the transferee can be in place for the opening. That order is backwards: the petition should be filed when the payroll evidence, the ownership chart and the US operating records are complete, which for most small companies is a quieter month. Build in time for a request for evidence, which is common where the supervised layer is small, and do not schedule the transferee's departure from her Canadian duties until a decision exists.
If the US site genuinely needs someone before then, the company can send a person for permissible business activities that do not amount to productive employment, but that is a narrow allowance and it should be examined for the particular tasks intended rather than assumed to cover a working season. Hypothetical example: a fencing-equipment manufacturer wants its plant director in the United States before a planned acquisition closes. The transfer cannot be planned on a role or relationship that does not yet exist.
Put closing documents, ownership updates, payroll proof, premises readiness, and petition preparation on one calendar. The first review should identify what is true at filing, not what management expects later. Trouble follows when an announced transaction is treated as completed and the petition has to explain why the corporate record does not match the proposed assignment.