

Double-checking is recommended as the season start can sometimes be slightly delayed and not happen precisely down to the second. There will be a system message displaying in chat, a pop-up notification on your screen and you can also check your friends list for seasonal heroes if you're unsure. Seasons always begin on a Friday at 5PM server time (CET in EU, PST in NA) and you can prepare the run in a way that you have the rift guardian spawned and kill it immediately when the season begins. However, don't complete it(don't kill the Rift Guardian) before the Season Start notification has popped up.

You don't actually need to have a seasonal character to collect the rewards from a Challenge Rift there later. You can start the Challenge Rift before the season begins, which will give you a Challenge Rift Reward Cache containing a good bunch of crafting materials, 475 Blood Shard s and 5.1 millions Gold to kickstart your leveling process.
