[2022] 100% XP boost from season pass rank 19 activates immediately after the round it is unlocked. If you want to save it to double your first match of the day bonus you have to stop playing until then

Someone please correct me if I'm wrong - but the "First match of the day" bonus does not get boosted by the XP Consumable items. Only in-round actions get boosted. In the End of Round screen it should highlight all boosted XP in yellow, and the "First match of the day" XP should remain as its default colour.