It is not unusual for young children to have a bit of an emotional wobble at the start of the day but generally they will be fine within a few minutes of being in a classroom with experienced adults who are used to working with such young children. This wobble can come at any time and so so not be alarmed if your previously settled child suddenly becomes upset at the start of the day. There are usually simple reasons behind this, such as being really tired, not feeling very well, worrying about something at home and in very rare occasions something at school. Again, make sure you talk to your child's teacher to share any information about possible causes and to work together to settle your child.
Tip: At the end of the first day, and for the whole of their first week tour child will probably be really tired. Try not to keep asking about what they have done as children will usually volunteer information about the days activities in their own time. If you are curious then ask 'What did you really enjoy today?' as this is less intimidating that 'What did you do today?' and is also more likely to get a response, even if that response is simply 'playtime' or 'lunchtime'!