Mandatory nature of cooling-off and strike-vote waiting periods practice question

The Progressive Workers Union (PWU) is the exclusive bargaining representative of the rank-and-file employees of Harvest Mills, Inc. (HMI) in Bulacan. On December 1, 2023, PWU filed a notice of strike with the Department of Labor and Employment (DOLE) alleging HMI’s failure to pay the 13th month pay. Six days later, on December 7, 2023, PWU staged a strike. The following day, December 8, 2023, PWU submitted to DOLE its report on the results of the strike-vote, which was conducted on December 6, 2023. HMI filed a petition to declare the strike illegal for failure to observe the required cooling-off period and the seven-day waiting period after the strike-vote report. PWU argues that the requirements are merely directory and that the strike was justified by the urgency of the workers’ grievances. Should the strike be declared illegal? Explain.