After 8hrs of travel the Force March rules (PHB p. 181)
Constitution saving throw at the end of the hour. The DC is 10 + 1 for each hour past 8 hours. On a failed saving throw, a character suffers one level of exhaustion.
Thus can I time some spells (Bless, Resistance, etc.) so that when the end of the hour comes up I get the bonuses on my save?
I believe I can, but would like confirmation. Please include citations.
Best Answer
Yes.
I don't think you're going to find a nice single citation the way you'd like. However, Forced March is just movement. While you are walking, there is no rule preventing you from using a spell while walking or to pause walking for 6 seconds (1 round) to cast before the time is up.
The Game is a Series of Turns
So, here's the deal:
From the combat section of the SRD under "combat" we find the above. It goes on to talk about bonus actions and reactions.
Every 6 seconds you get an action, a bonus action (if granted by a spell or ability), a reaction and movement. While travelling, you generally only care about the movement -- but each round of the 8 hours you could spend an action doing something while you walk. That thing, especially when you start to feel tired (before your DM makes you roll) you can cast a spell. Generally, while walking we skip asking every 6 seconds worth of travel, "What do you want to do?" because that would be really boring.
So, each 6 seconds, your character can take one action to cast Bless or Resilience or light or any other thing you have slots for and is prepared. Your best option is to describe the plan to the DM before you get that roll. If the DM asks for the roll and then you say, "Before that I cast Bless", the DM could rule it was too late -- you didn't realize how much time had passed.