JOIN multiple tables with WHERE clauses

I have a master table (Project List) along with several sub tables that are joined on one common field (RecNum). I need to get totals for all of the sub tables, by column and am not sure how to do it. This is a sample of the table design. There are more columns in each table but I'm showing a sampling of the column names and values to get an idea of what to do.

Project List
| RecNum |
|      6 |

WeekA
| RecNum | UserName | Day1Reg | Day1OT | Day2Reg | Day2OT | Day3Reg | Day3OT |
|      6 | JustMe   |       1 |      2 |       3 |      4 |       5 |      6 |
|      6 | NotMe    |       1 |      2 |       3 |      4 |       5 |      6 |

WeekB
| RecNum | UserName | Day1Reg | Day1OT | Day2Reg | Day2OT | Day3Reg | Day3OT |
|      6 | JustMe   |       7 |      8 |       1 |      2 |       3 |      4 |
|      6 | NotMe    |       7 |      8 |       1 |      2 |       3 |      4 |

So the first query should return the complete totals for both users, like this:

| sumDay1Reg | sumDay1OT | sumDay2Reg | sumDay2OT | sumDay3Reg | sumDay3OT |
|         16 |        20 |          8 |        12 |         16 |        20 |

The second query should return the totals for just a specified user, (WHERE UserName = 'JustMe') like this:

| sumDay1Reg | sumDay1OT | sumDay2Reg | sumDay2OT | sumDay3Reg | sumDay3OT |
|          8 |        10 |          4 |         6 |          8 |        10 |