You can certainly stop for the day. The things you have to watch are:

- Finish your tiles all in one line or row, not on the diagonal. That way you can check it with a laser or string to make sure everything is straight. Once it sets, you can't nudge tiles to get a good line.

- Depending on your pattern, you may have adjacent joints lap one, two, three or more tiles. The more laps, the easier it is to hide errors in height. When the tiles are wet, the installation of adjacent tiles lets you feel height differentials (lippage), and then adjust using hydraulic pressure of the cement as you lay the next run of tiles (tap the high tile down, or lift a low tile by tapping adjacent tiles).  

  When you stop for the day, that leading edge you have laid can come back to haunt you the next day when you start your new run because the lippage can't be adjusted. If your floor is completely flat, its usually okay, but if you are laying on a floor that's not perfect, it can be a problem. It also depends on how picky your client is, and what you are laying. Marble and granite can be polished down to remove lippage, but ceramic and porcelain don't work that way. Bigger tiles are worse than smaller tiles. 

  We install lots of tile, and its all got to be within 1/32 height to the next tile (otherwise you feel it underfoot when its done). You can solve this pretty much if you use a tile levelling clip system as you install. Just leave a height setting tile "clipped " in place along each both sides of the joint at the terminal line, just don't mortar it in. Next day, come back, slip out the tile and continue.

- Don't disturb the tiles the next day. Assuming you finish at 5 and back at 8 the next day, the tile mortar hasn't had enough time to set fully. If you start laying new tiles too soon, you may inadvertently disturb the terminal row. Let it cure for 24-36 hours. 

- Use a good quality polymer modified mortar to install. This will give a stronger bond and minimize the chances of popping tiles as you start and adjust the new course on day 2. Dyna Ceraflex 610 is my go to mortar. Or Flextile 52, but honestly, they are pretty much the same and the former is half the price.